Well, mostly it's preference - in that I prefer stories with Spike in them and I'm a bit picky about how he's written.

However, I have to admit to prejudices which mean that - due to one thing and another - there are certain 'ships I just won't read, no matter how great a write up the story gets.

Having said which, if one of my close LJ friends wrote a story with those 'ships, I'd read them, because friendship means more.