Gameplay is more important to me. The story, characters, and environment is what engages me, but the gameplay is what keeps me playing. With that said, I would not think very highly of a game with a crappy story unless of course it doesn't really have one and I'm just playing it for mindless fun to begin with. It also depends on the genre. For instance, I believe RPG's demand a good story because the formula of traditional RPG's; especially turn-based is very cut and dry.