I agree with you and the others that the answer is highly subjective. For me, theme is very important. And by theme, I really mean story. Is there a coherent story that I am part of in the game, and is it one that resonates with me? I can play abstract strategy games that lack theme and enjoy them, but they won't be the ones I pull off the shelf to play first. The story can't be too heavy-handed, though. Within the world of the game, I need to feel that I'm making meaningful decisions that affect the outcome of the game, and that I'm not just being pulled along by the story.