I saw this thread when it first popped up, and have been really wanting to reply to it, until I realized it was really difficult for me to fully formulate in my mind. Still is, by the way.
I think it ultimately boils down to what it always does for me in games; The story. Games are just more interactive and entertaining books for me, so the more a game suits that purpose, the more likely it is for me to enjoy it. So stuff like combat isn't of as high value to me, it's just there. Though if combat drives the story, or holds emotional interest through the story, then I'll love it. More specifically I like character building and universe building, exploring mythos and mystery (Dragon Age does a fantastic job with this for my tastes, such as Morrigan with the mirrors and Inquisition's whole foreshadowing and subtle hints to Fenris throughout the game, and its conclusion).
So... A living world to explore with character(s) I find interesting, as they're the viewport you have into the world, with schemes, mysteries and events that exists and develops there regardless of my existence, but recognizes my existence if I choose / find a way to interact with it, I suppose. I can't really find better ways of putting it down into words at the moment.
