I guess I'd have to say character development. Basically, Kotor2 was a game that made me actually pay attention to, and like (or dislike) the characters. Most RPG's I've played, even if I really enjoy the game, characters are just pixels to control to give me an advantage in a battle or there to give me instructions; they affect me emotionally in no way whatsoever. So any that do, are exceptionally memorable to me.