Adding mods to games is so much fun. I've got about two dozen mods on Fallout 3 and they really change the way the game plays. Things like green bushes and trees, extra companions, a gun that makes people dance... endless fun!
The AI in Oblivion/Skyrim needs a bit of tightening up I think. In Fallout 3 if you kill someone the whole town goes berserk. I guess bounties in general need a bit more work, for them to be more realistic. Rather than: steal an apple - guards run over and attack if you dont pay them 5 gold!