The first couple of thoughts that come to mind (I imagine there will be others to add)…
1) The whole “Rock, Paper, Scissors” idea of balancing classes. I do not like that a certain class is at a significant disadvantage when facing another class simply because of the class abilities. Give classes abilities on a move, counter-move system, where a player who knows his/her class (and more importantly knows the opponents class and abilities) can use skill, timing, baiting, and anticipation to win a fight. Whatever your opponent does you should have something that can counter or defend against it; and vice-versa.
The reality in some games is that when a mage and a warrior face off, the warrior is dead (mage will kite him all day). When the same mage runs into a Ranger the mage is doomed (can’t kite the ranger!). That is no fun and in my opinion, is lazy class design.
2) Repetitive dungeons that have no challenge whatsoever. Some games have players run the same dungeons over, and over, and over… even to the point of people in LFG wanting “speed runs” of a dungeon because it poses no threat or challenge, they just want the points to get whatever reward is available for somehow managing the will to run the content 59,000 times.
Remember the good old days of needing crowd control and smart pulling to survive? Or maybe some random variation to the mobs or script that keeps every run from being the exact same.
3) Invisible walls. Yes, a bit of a personal pet peeve, but I hate invisible walls that block off visible paths or areas. Open world and exploration should be exactly that… if I can see it, I should be able to go there.
I just joined up today, so these are just what popped into mind at the moment. I will surely have more (hopefully better) feedback after some thought.