The bad stuff was the poor path-finding and the poor AI (archers moving to shoot, even though they don't need to), the unresponsiveness due to the faux-turn based combat (you know when your character doesn't start to act until the next "cycle" in the turn-rotation). Generally the lack of a good implementation of AD&D. ToEE, albeit being d&d 3.x, was utterly amazing in comparison to the combat system of IE-games.
And also how the games lacked non-combat skills. They were all slash and hack games; except for PS:T, which had plenty of stat-checks in dialogue.
And a lack of choice and consequence; responsiveness in the game world.
The good stuff was the beautiful level design, the richness of items (and descriptions) and characters. One of the things that made BG was the fact that you could walk up to the edge of a map and cross over to the next. There was no need to talk to some entitled bitch in a tavern to be able to travel to a rural keep; the game world was almost all there right from the start for you to explore. Nice~