1) I hate respawning monsters. If they respawn, there has to be a very good reason why this happens, and even then, there has to be a way to stop the respawning.
2) I dislike backtracking without being able to fast-travel to and from specific locations. NWN and NWN 2 are already slow as they are.
3) I dislike easy combat and underwhelming boss battles. Like when there is an organized group of intelligent enemies in some location, but for some reason they do not have the concept of security, and don’t have established watch routines (but have very dim vision and are pretty much deaf, apparently), so you just walk from a small group of enemies to a small of group enemies, picking them off one by one without the rest of the group reacting.
I want to have to prepare for fights, to look for enemy’s strenghs and weaknesses, and to mind the type of terrain, and the type of monster population, lest my party dies. I want enemies in the whole area to become aware of my presence when I make too much noise, so they group up, take defensive positions and have their spellcasters cast all buffs and summoning spells that they have before I even get near them.
4) I dislike how generous a lot of modules are with resting. Not being able to recharge your spells until you go back from a dungeon (and losing all progress if you leave the dungeon without clearing it) is the way to balance spellcasters (along with other PnP stuff like additional cost in time and money to prepare and cast spells). Without it, there is no reason to play mundane classes other than for some strange kind of a challenge.
5) I dislike when battles do not follow the obvious logic of power scaling. A textbook example of this is the Blackguard module, where you fight heroes and celestials and ancient evils, but then you face more challenging battles when you are sometimes attacked by a mob of townsfolk, who are wielding Pitchforks of Godslaying +6, apparently.
6) I dislike towns where nobody cares that you steal or break things. Unless you want to make a point that the location is a grim and lawless place, you should make the guards actually do their job, and the people to be more aware of the player’s dangerous behaviour.