I play through an RPG primarily because I want to know what happens next. I want to know who the bad guy is, where did he go, whether X and Y get together in the end, if so and so is really dead, etc. Given that, I think it's fair to say that my main focus and driving motivation is the story, even if it wouldn't necessarily hold up on its own. There's a supporting cast of features that prop the story up, but it's still the main attraction.

Put differently, the story is the skeleton. When I'm running my character around a room, I don't go to the next one because I want to see what lovely graphics have been put there. I don't go because I want to hear the characters talk for a while, about whatever they feel like. I go because I want to see what happens! To me, that's proof enough.