I think that what has the biggest impact for me in terms of gameplay which City has done right, while so many others are awful at, is the strength of Controls and Debuffs. A Controller, Defender, Corruptor, Dominator, and Mastermind feel meaningful and significant in combat because there is a definitely and tangible benefit to laying down status effects. The only other game where I felt that status effects were even remotely impactful was League of Legends, but that game has a whole host of other problems with it (not least of which is that its devs only care about their precious e-sports nonsense).
Further, I think the reason I wasn't too thrilled with City of Heroes PvP was because all the Control and Debuffs were reduced in effectiveness during PvP. At least while I was playing, back in the day. Not sure if that ever changed.
Of course, that's not the only reason CoX was so great, but it's definitely one of the biggest elements as to never being able to find satisfaction in any other MMO for me.