I read stories because of the author, because of the pairing (or one part of it), because I am totally bored and don't want to work (then I read almost everything, or at least start).
There are some themes I am not interested in (incest, rape, really dark fics), or I simply cannot stand the pairing, e.g. I have a strange disliking towards Buffy, but love the scoobies.

And I love to read cross-overs, because it is not canon and will never be, and my preconceived notions about background and story line within the fandom are totally irrelevant, and it is a playground for my fantasy. And often I have one fave character and want her to make out with my other fave character who happens to be on another show.