immersion, obviously, but a VERY big part of that is story, or specifically narrative. at the very least the game needs a compelling narrative; if i cant find a reason to keep going, i quickly get bored.
like crackdown 2 so completely failed to do, every game needs something to drive the action. a great big murder playground is great and all, but unless im given some kind of believable reason to be there exploring it....well, like yahtzee said, you just end up climbing the tallest thing in the game and hurling yourself off of it, and then your bored.
this is why i love MGS and final fantasy; in retrospect theyre quite mediocre games with quite convoluted plots, but their so good at keeping me invested/ immersed in the experience that i dont even notice. contrast with say..turok:evolution (thats the best ive got off the top of my head...), where despite ridiculously varied and entertaining weapons, the game quickly falls flat because the 'go through level, kill stuff' formula quickly gets old; the plot is so terrible you dont feel your in the levels for any reason, or care to kill things for anything beyond the satisfaction of dismemberment. a good narrative can cover for so damn many gameplay flaws, and likewise a bad/ non-existent narrative can make all the entertaining gameplay in the world quickly get boring.