Playability. A game can be playable in a number of ways, but if I'm not enjoying the system enough to get immersed in the basic gameplay, I'm not going to get far enough to care about the story before I get annoyed with it. That doesn't mean being easy, or conforming to set mechanics, it just means being inviting enough to the player, and actually achieving what it set out to accomplish.
(Besides which, I'm one of those people who often forgets the story in order to merrily run about my little sandbox world poking everything until it explodes or shoots me).