Well, let's start with what I hate:
- Over-emphasis on grinding: The game's length is artificially stretched between content updates by making it take forever to get anything done, whether it is getting a good item that costs a lot of gold or just leveling up. Even most content is a grind, as most MMORPGs have a lot of tedious fetch quests and kill X of Y thing.
- Pay to win: This only applies to MMORPGs that have microtransaction models. I played Runes of Magic for 3 months and I used to see mages sporting better armor and physical stats than a knight (when the whole point is that the knight is RoM's tank class and the mage is the glass cannon)
About the only thing I love about MMOs is character persistence. It feels good to know that you can return at any time and play the same character in new content. However, the negatives for me are far too many for me to really get into another MMORPG.