Favored/Hated gameplay mechanics across the franchise.

I'm sure there's plenty of things I can say but off the top of my head...

FFX - Summons are my favorite in the entire series. They don't feel like a "trigger long cutscene for massive damage (which ends up being pretty terrible late game usually). I like that they come out and you can use them and actually put work into strengthening them.

FF9 - I really liked the ability learning system where if you wore the piece long enough you unlocked that ability to use without that piece of gear.

Final Fantasy IX: Trance

God it was annoying to have someones Trance triggered by a random mob two minutes before a major boss encounter.

Probably forgot a few but these were the first ones that came to my mind:

Favored:

Lightning Returns: Monster types can go extinct if you defeat enough of them. Incredibly satisfying especially since you get to fight a stronger "Last One".

FFX: Blitzball. Somewhat controversial, but I am impressed with how much effort was put into this mini-game and I thought it was a lot of fun.

FFXIII: I loved the new take on ATB combat with focus on speed and preparation.

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Unfavored (hate is a strong word):

FFXV: The regular Regalia. It is so frustrating to get the car without being able to properly drive it around. Fortunately the offroad version fixes this.

FFVIII: Junctioning and enemies being scaled to your level. I feel like their vision for these mechanics could have worked but they are neither well executed nor explained well in the game.

Tactics: "Random" recruits in your party. I found them a bit weird, because your force was never big enough to really feel like a huge army, but you usually had half your group consist from these faceless mercenaries anyway.

Final Fantasy XII's Chain mechanic is something I really like. Defeating several of the same enemy type creates a 'chain' which increases the likelihood of foes dropping rare loot. It adds an easily understandable element of strategy and reward to grinding for drops, which helps since monster loot is your main source of currency in XII. Hitting a high chain and seeing giant gold medallions popping out of half the enemies is really satisfying.

I think Final Fantasy X has the best "Limit Break" type feature in the entire FF series with Overdrive. Limit Breaks in other titles either replace the attack command (such as in FF7), require you to be at critical HP (FF8), or - as you've noted - are sort of uncontrollable, like in FF9.
Overdrives, on the other hand, can be stored up and used strategically. You can go into battle with a full set of Overdrives on deck and unleash everything you've got. Giving Aeons overdrives is also pretty neat, and Seymour even gets an Overdrive for the single fight he's in your party. You're also encouraged to manipulate how often Overdrives happen through the different overdrive modes, weapon and armour abilities, and sometimes intentionally taking damage (in the case of the Aeons' Boost ability).

I dislike Final Fantasy VIII's Junction System, in which magic can be 'junctioned' to your stats in order to increase HP, Strength, and so forth. On paper it sounds like it'll make use of magic in battles much more tactical, but in reality it makes me reluctant to use magic at all out of fear it'll make my characters weaker. It's also heavily exploitable: just by drawing the right magic and learning a few GF abilities, I can get a huge stat boost without levelling up traditionally or progressing through the story. It doesn't feel earned in the same way.
This also leads to the other thing I dislike about FFVIII - the fact that all enemies' stats scale up as you level up your characters. I really like reaching the point where I'm over-levelled for an area and I can swat enemies away that were giving me trouble a few hours earlier. With VIII, that either doesn't happen because the enemy levels up with you... or it happens because you junctioned Ultima to your strength stat.

Juction system. I like the customization it offers but I HATE drawing magic. I HATE refining magic. I HATE having to play cards/card mod for magic and items and I Hate having to get 100 of a magic for max benefit.

A junction system where spell number didn't matter as long as it was juntioned allowing you to stock and use magic at your leasure would have been a beautiful system.

FFXII - The Gambit system was amazing. I thought I'd hate it and that it would basically allow the game to play itself. But playing around with Gambits and finding the best strategic combinations for any given situation was quite a bit of fun. And there's so much going on in combat that you still have to pay close attention, which the Gambit system allows you to do because you can take a step back and observe everything going on around you.

FFII - Grinding is tedious. Nothing will ever change that. What made FFII's progression system so unbearable to me was that you had to micromanage every single stat. Every single weapon. Every single spell. I've come to realize it's the same thing that I don't like about Bethesda games. The idea is to make the experience more about tailoring your stat proficiency to your playstyle, but instead it's a system that practically requires you to exploit the mechanics through hours of repeating the same action until you hit the desired benchmark. And then you move on to the next stat and figure out where and how to raise it to where you want it. Though in FFII's defense, the remakes made that system progressively less tiresome with each version.

EDIT - (Dis)Honorable mention to FFXII's random loot. The game helpfully gives you an indicator so you know when a chest is near. Then it punishes you by not giving you certain items if you don't have a specific accessory (except for when the rare item WON'T SPAWN if you have that accessory equipped). And you can miss out on the Zodiac Spear by looting "forbidden chests" that the game doesn't tell you about. For that matter, the game doesn't tell you how looting chests works at all. I spent half the game thinking chests all contained random garbage and had no idea how the mechanics of loot worked until I finally broke down and consulted a guide. And then I wasted DAYS trying to get rare loot to spawn and it never did.

Favored:

FFXIII - The new take on ATB. Battles are lightning fast. So much love for this battle system

FFXIII - Paradigms. Love being able to switch to fit the needs of a battle on the fly

FFX - Blitzball. Best mini-game in the series for me

Unfavored:

FFXIII - Being progression locked by story. By far my favorite game in the series, but my favorite thing to do in JRPGs is to battle and get stronger. The game telling me I have to give up on that until I move forward is annoying.

FFXIII - The party being split for so long/being unable to choose leader in much of the early/mid-game

X

Good

I agree about Overkill.

Best implementation of ultimate skills.

Bad

Dark Valefor and Shiva are really badly placed.

The damage formulas are badly balanced, making magic almost useless in the end-game.

XII

Good

The Gambit system is just sublime.

MP regeneration is incredibly useful but the rate is balanced such that you can't spam the most powerful spells.

Bad

The amount of RNG abuse in the game is ridiculous. Even HP and MP growth when levelling up is random.

The implementation of the loot system and bazaar means that you'll either be overlevelled when you do get the gear without New Game Minus/Firefly or it will be unlocked long after it's useful.

XIII

Good

MP isn't a thing! Magic is well balanced to compensate.

Bad

Crystarium is pointless. It would be better to have the roles level up in a standard way but separately.

Sidequests are wank.

Junction.

shudders

Like:

Old school FF in general: World maps and airships. Seriously, why don't the modern FF games have these? DQVIII was a modern JRPG on the PS2 and it managed to have a world map and allow you to fly around late-game. I suppose FFXV kind of does, though it's not really the same. I miss having a full world to explore, the whole damn globe, and being able to fly freely around that world and land about anywhere I wanted.

Final Fantasy V: The Job System. Specifically, one that allows learned skills to be equipped when using other jobs. This is a brilliant way to handle character development and it's no mistake we've seen the system adapted in spin-offs like FFT and Bravely Default over the years.

Final Fantasy VI: Everyone having unique classes while also being able to learn magic. To this day, no other game in the series has quite reproduced this set-up. While we're on that topic...

Final Fantasy VI: Relics. Again, no other game in the series has quite matched how the relic system allowed for customization of your party members, in part because either 1) accessories seem nerfed in many games compared to FFVI, or 2) you only get one accessory slot... or none at all.

Final Fantasy IX: Learning abilities from equipment. This was a great idea and I'd love to see it referenced again in the series.

Final Fantasy X: Overdrives are my favorite limit break type because 1) they're all unique to the characters and 2) you can save them and use them when you want.

Final Fantasy XII: The hunter's guild was great because the hunts actually had stories that expanded the lore attached to them, and your targets were unique monsters. Compare this to FFXV, where the hunts are mostly just busywork MMO-style hunts that are usually for the same trash mobs you're killing all the time anyway.

Dislike:

Final Fantasy III: The lack of save points. I know this game is old, but jesus, that stretch through the final areas is murder. Especially when your DS freezes up after you've killed the third boss in the dark world just before the end and you lose an hour or more of progress.

Final Fantasy VIII: Enemies leveling up with you was broken as hell, especially because their levels were tied to their item drops... and often, once they leveled up it was harder to get the items you needed to reforge your weapons. Related....

Final Fantasy VIII: Every level requiring 1000 EXP. If higher levels had required more experience points, I feel the leveling curve would have worked a lot better than it does.

Final Fantasy XIII: Only being able to control one party member. Fuck this. Fuck it so goddamn hard. I want to control my whole party, every action, every turn. This is such a goddamn terrible gameplay decision. Speaking of which...

Final Fantasy XIII: Only having 2 characters per party for so much of the game. This was also just absolute shit top to bottom.

Final Fantasy XV: It's a goddamn action RPG. I am so fucking tired of action RPGs. They're goddamn lazy game design. Give me a menu-driven, turn-based/ATB system any freaking day. It makes me sad that FF has gone this route, and I won't be buying future games in the series until well after they've released and the hype has died down so I can hear if they're actually worth playing or not... and that is in large part because of XV being an action RPG instead of a real JRPG.

The good ones:

FFVI: The World of Ruin was a very unique way of handling the pace of the game. There was a big opportunity to have more insight on each character on that horrible reality, and all completely up to you. Great way of tackling a story with so many heroes.

FFVII: The Materia system allowed me to do some crazy shit. Best combat mechanic in the franchise.

FFVIII: Triple Triad was hands down the best mini game in the franchise.

FFXII: The Active Dimension Battle, as they call it, with emphasis on the Gambit system was one of the best decisions they made. Seeing the actual monsters on the map brought life to it and was an astounding improvement over the saturated and often times frustrating random battles. Gambits also made me not hate ATB.

The bad:

FFVI: The Dragon's Neck Coliseum's mechanics were a horrible idea. What were they thinking?

FFVIII: The Junction system was a mess. Why gimp yourself by using the stat boosting magic? Couldn't this be easily remedied by keeping MP values and some tweaks, like empowering the junctioned spells? An interesting idea, poorly executed.

FFIX: Tetra Master was hands down the worst mini game in the franchise.

Like; X: the sphere grid. It's probably my favorite leveling system in any game ever (well besides path of exile which has a pretty similar system I also love). I love the choices you get to take your character on, but you have to invest more (lock sheres) if you don't want them on the "intended" path (ignore this point if using expert grid). And it's just fun unlike a basic stat allocation like in most games.

Dislike; II: the skill/spell leveling system. While the idea that as you use stuff it becomes more power does seem like a fun cool idea, I don't like it in play because it disincentivises trying new abilities, or requires a massive grind to bring new skills up to par.

Dislike; multiple titles: unskipable cutscenes. Just wastes your time if you are replaying the game, or if it's right before a fight that you can lose at .

Been a while but specific mechanics that stuck out:

7: the materia system,probably the best leveling,ability mechanic I've ever seen in an rpg though elder scrolls "doing the thing" is close.I really really hope it is as good or better in remake.Also on secret characters,love them.Same with secret ultimate weapons,though I must say thank god for the internet,the first time I made much great use of it.

9: the learning abilities from gear was great,it made even the smallest bandana valuable and you were always working toward something becoming ever more powerful.The minigames were fun as well,even the skipping which I loved/hated.Only made it to around 83 I think but that was pretty satisfying and was enough for me (and my sanity).The sidequests,so good,the series best,tracking down all that loot and eventually finding secrets in secrets at the end of it all,as someone who loves these diversions this was the pinnacle.

Lightning Returns: the battle system,worked really well.It was fast and responsive,they did a great job of refining 13s battle system.I don't know how they are going to end up doing things in 7 remake but if it ends up near this I'd be happy.Its a great mix of old and new.

15: the photos were actually pretty good,especially since they were unique.I liked seeing everyones best shots.

Hated mechanics:

13: linearity followed by forgettable side missions when freedom was finally given.Also very weak ability tree and terrible ultimate weapons.Those,adamantoises,my god.Fun fight,now do it for 20 hours...

15: pretty much all of it but to start with.Driving,got old fast,the music from old games was nice though and probably my favourite part of the game.This was skippable,sadly though the load screen nearly took as long.Combat,the most erratic,chaotic combat of nearly anything I've played.In the end it seemed the most effective stategy was to spam warp and throw in some magic potion? Things.Minigames,not many here,pinball wasn't great,chocobos were meh,fishing was alright though.Side quests,felt like a bit of a checklist without very meaningful rewards.That’ll do I guess,I think he's had enough.

FFX- I love the summon system in this game. I also love that all equipment in the game (except celestials) were all "side-grades".

FF8- I don't hate the idea of the Junction system in theory, but drawing is very tedious.

IX: Trance. Horrible. Just what were the developers thinking? No control over it, potentially useless (Quina, Garnet) and it could be triggered by the last attack the enemy does before dying, just to add further insult.

However IX Also had the best skill learning system.

Favorites:

• The time mechanics in Lightning Returns

• The crystarium in XIII, especially it being limited by story progression

• The paradigm system in XIII

• Garment Grid in X-2, although I have some issues with its implementation

• The costume stuff in Lightning Returns

Dislikes:

• Grinding

• Minigames

• Exploration/Open World

• Old-school ATB

• The combat in pretty much all the games pre-XII

• The way that most of the games other than like the XIII trilogy have major issues of gameplay-story discontinuity

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Good: visible monsters that you can choose to attack or avoid, i.e. XII or XV

Bad: random encounter whoosh every 10 steps, even more so if the overworld/dungeon music restarts after every battle, i.e. everything before XI

Depends: monsters spawning right in front of you - depends on pacing

Good: strategic systems like X's turn based and XII's gambits

Bad: early "everyone gets a turn" systems, i.e. I-III

Depends: ATB systems - depends on other mechanics

Good: Characters having specific roles, i.e. I, IV, IX, X, XII (IZJS/ZA)

Bad: "Build your own guy", i.e. II mains, III, V, VIII, XII (OG)

Depends: Intended roles with customization, i.e. II guests, VI, VII - usually gives too much leeway and becomes "build your own guy" with restrictions

Good: easily accessible commands in turn-based and ATB systems, either executed right away or with a short delay

Bad: ATB with long or multiple menus, i.e. most magic and item menus. Can be somewhat mitigated with cursor memory option or wait mode.

Worse: Half the special ability gimmicks in VI - scrolling through Gau's 40+ rages, waiting for Cyan's bushido to charge before you could even command him to use it, Sabin's fighting game-style inputs which you could easily input wrong

Good: Recharging MP or affordable ethers so you can actually use your magic.

Bad: Anything that encourages magic hoarding, i.e. selling ethers for 500x as much as potions; dungeons so long that you run out of MP halfway through; the junction system in VIII

Worse: Restricting characters to magic roles by either making them physically weak or limiting their weapon options, and then making even end-game spells less damaging than mid-game weapons

Favorite mechanics - Gambit system from FFXII and Character Swap system from FFX.

Worst mechanics - Trance system from FFIX and Junction system from FFVIII.

I really love how deep the customization in FFX is, even though I usually hate it when every character can become interchangeable once fully mastered.

You can create weapons and armor exactly how you want, you can have your characters become exactly what you want and you can even control how your character's overdrive gauge will fill. Plus, overdrives are very visually appealing.