Wow, that's a real toughie! I'm gonna have to say story. I may have times where I skip through cut-scenes when able, but I still consider the story the vitality of a game. If there's no story or interesting characters, I find the action kind of pointless. Then again, I mostly play RPGs and simulation games- but it might be because those are what I play that I think the story is so important. The actions of creating in a simulation game mean nothing if there's no at least basic plotline to help you understand why you're working on what you're working on.