I'm gonna have to vote NPC interaction. Just thinking of games like Torment, Fallout2 and BG2….

Quests.. Story.. Choices and consequences.. they pretty much are NPC interaction to me.

Exploration from games like Morrowind and Two Worlds a close second.

also, I started thinking:
When is combat most fun? In which game has the combat been so much fun that I could play it just for that?

…….

Fallout 2… and.. uh…. no, that's it.