Taking Advantage of the Terrain [PvP Tips]

Collision Awareness
This image shows a standard Guild Wars starting position.
#1 is in the lead position, able to charge in or retreat back. #1 will stay on the lower half of the bridge should they decide to attack.
#2 is in a supporting position, following #1's lead whether to stay or go. If they decide to attack, #2 will move to the upper half of the bridge, so that #1 is given room to retreat back safely if needed.
#3 is in the secondary support position. Far enough away from #2 to avoid both of them getting caught if the enemy attacks, but still able to see bridge to determine if the enemy has crossed over. On the attack, #3 will stay on the lower half of the bridge to allow #2 to retreat back if needed.

What: Player models have collision boxes, including teammates. This means you cannot walk through them and you must plan accordingly, so that you do not unintentionally body block each other.
Where: Mainly on the bridge in the Guild War arena.
Pro: You can create a strong choke-point for your ranged teammates, but your enemy can also do the same.
Counter: Slip past the enemy line and go deep to force the enemy to chase you.

Lower Ground
In this image, #1 is able to see the entire body of #3, but #3 can barely see the top of #1.

What: Due to how the camera is positioned, players often cannot see what kind of horrible fate is awaiting them from below.
Where: Extremely common tactic used in Brawl and Guild Wars, you already know these spots!.
Pro: It is a very advantageous position to hold; it often forces the enemy to check with a skill or risk getting ambushed.
Counter: Use a skill to dive right into the area or dodge away in an attempt to bait an attack.
In Guild Wars, you can have one person traverse the bridge as bait and have the other 2 ready to rescue, hopefully you'll catch all 3 of the enemy team in one spot!

Shadows
As this image shows, the shadow extends much further than the actual player model.

What: Due to the way that shadows are drawn, the player on the left will always see a player approaching from the right first. Specifically, the left player will spot the right player's shadow.
Where: Duel and Guild Wars are where this is most applicable.
Pro: If you spawned on the left side of the map, know that you will be able to see the enemies' shadows before they can see any part of you. If you know they are stationary, you can inch up and spot them first, provided they are not utilizing any other terrain advantage such as the lower ground.
Counter: Kiki's teleport is not a smooth movement like the other characters' dodges. Fast moving skills are often too quick to be able to reacted to based on shadow alone.

Slowed Movement

What: Running at an angle to the wall will slow your movement.
Where: Duel, Brawl, and Guild Wars.
Pro: You are able to inch your way forward instead of going full speed ahead. You can use this to spot shadows without going too far in to reveal yourself.
In Brawl, you may hear Kiki using her magic circle, but if you run down without slowing down, you could run right into the fire. Instead, you can grind yourself against the wall and slowly move forward to see what the heck is going on first.
Similarly for Guild Wars.
Counter: This is usually used to counter the lower ground advantage and only in specific circumstances where the other player is already occupied with another skill, unable to react to the walker.