I think it's the story, and whether or not it's believable.
That said, obviously genre is important too. A fantasy book with magic, dragons, and gods is not the slightest bit believable. But the story itself might be.
I read a book once which was set in a fantasy world, the world itself was obviously not believable. The story itself was quite good though, up until the point where a 10 year old buy was introduced. A boy who knew more of the world than the other characters. A boy who was far more intelligent than the other characters. A boy who could calculate advanced equations in split seconds. And a boy who had never left his home town, who had never recieved any kind of education.
That sorta ruined the story.
So, the story itself has to be believable
