If you want a simple list: - Quested leveling and story-telling - A good character customization system - Trinity-based combat design with emphasis on parity within roles while maintaining diverse gameplay between classes - Quick action/reactions by the developers when players do things not intended

The long version:

People complain about quests and mundane things that go with it, but conveniently forget that quests are part of something rather important (if you care about the whole RPG bit of the MMORPG) - context and world-building. Sure, if you don't give a shit about story it's not gonna matter to you, but if given the choice between your standard grinder (Tree of Savior, Black Desert Online) and a game with quests and attached storyline (WoW), I'm going to go for the latter every time.

The trinity is another aspect that gets bashed, yet people neglect the fact that a) the trinity gives classes structure and parameters that make sense most of the time, and b) prevents classes from devolving into one-trick ponies.

That said, parity is also important because if you have two different tanks or DPS but one does better than the other, you have a big problem in your hands. Approaches to gameplay should be where the player sees the differences between class A and class B while still having very similar-if-not-the-same results regardless of which class the player picked.

Quick reaction is there mostly for personal reasons. I hate it when people do unintended things that lead to a change in overall player behavior (doing unintended things for fun like the tamable ghost wolf in WoW is a different matter). I hate it even more when the developers sit back and do nothing about it. There's a ton I could mention, but a good example of dev reaction is how WoW dealt with the platform bug in the Heroic Lich King encounter. A bad example would be how the FFXIV devs sat on their hands on the enrage exploit for Turn 2 of the Binding Coil of Bahamut.