Let's take it in order.
BG1 :
1. The Area Style. In BG2, for instance, all the areas are somehow related to your quests and also you can't reach them unless someone gives you the proper task and marks the location on your map. That was a dumb idea. For instance, you should have been able to travel to Umar Hills or Trademeet even though you did not meet their emissaries. The fact that you are able to go only where you have a task to carry out is quite annoying. On that map from BG2, if Bioware had respected the AD&D geography, there should have been some other cities where you could have gone. Adding wilderness areas will also be nice.
BG2 :
1. NPC's personalities. The BG1 NPC's acted just like dumb dummies.
2. The party (that's together with BG1 )
NWN :
1. The perspective. I found that aerial view from NWN far better from that "over-the-shoulders" view from his competitor, Morrowind.
2. Henchmen personalities (original NWN and HotU). Maybe it sounds strange, but I shall explain : the NPC's interaction had a problem. They had comments to your decisions and between each other, but there was nothing revealed about their past (except for the romance, because their stories were part of flirts). There are some NPC which we don't know almost anything about : Minsc, Yoshimo, Korgan, Haer-Dalis, Mazzy, Nalia, Valygar and Edwin. Exception are Keldorn (due to his banters with Anomen), Cernd and, of course, Jan. But again there are only general things. Although you can't control them, you can get a far better insight into the henchmen's history from NWN.
To drop :
Well, here NWN has the most flaws. Obviously, the henchmen idea, the linearity and the map style (the maps in NWN give the impression of a table for toy soldiers).
Edited by Feanor, 20 December 2004 - 02:35 AM.