I like the idea of morality itself. It makes you feel more immersed in the game, and puts you in the shoes of the character.
I don't like this whole "KILL EVERYONE AND YOU ARE EVIL" or "SAVE EVERYONE AND YOU ARE GOOD" nonsense, though. I prefer moral grey zones - killing one man in exchange for the life of another; that blood magic demon summoning from Dragon Age: Origins; things like that.