storyline mostly but i'm a little divided in this category.
for example, some games like Bioshock, F.E.A.R., Alan Wake, and Red Dead: Redemption that engross you in the story and weave it into the fabric of the gameplay always have my attention as long as the gameplay is good as well. but on other occasions, the story could be or metaphorically is completely non-existent and i wouldn't care, games in this category include Left 4 Dead and Painkiller
so basically, story has to be done right, or it's painful, the only example i have here is Bioshock 2, where the story is just outright pointless and stupid, but seem to think otherwise and won't stop shoving narrative down our throats.
And one more thing that makes a difference, controls. This doesn't really present a large issue for me, as most games i play have understandable fluid control schemes. the catch, though, is when i'm switching around between multiple games with completely different control styles. For example, at one point is was alternating between playing Bioshock, Resident Evil 5, Fallout 3, Red Dead: Redemption, and Left 4 Dead 2, resulting in understandable confusion.
