Nostalgia setting in

Hey guys, may be some silly questions but maybe I can get some simple explanations. Where does classic sit? I hear TBC is coming to classic and debating returning to play. My best days in game were in TBC an WOTLK. Is it worth coming back? I've read alot about SoD but it doesn't seem like it would be the game I enjoyed playing back in the day. Just looking for opinions between the options as with all the new changes. I guess I just don't understand what's going on anymore. Classic era, SoD, vs classic? since I stopped playing in the original cata release I dont know what I'm looking at with the mess of updates of each corner of this game. Is era different from classic? I liked the old ways, so is just classic what I'm looking for? Will hardcore classic be any different?