The thing is, books aren't read in a sterile bubble. We all have mental maps, based both on real life and the other books we've read. For someone who reads a lot of urban fantasy, Twilight doesn't present anything that hasn't been done before, but to someone who isn't familiar with vampire books, it could open a whole new world. (Or, y'know, shut it firmly depending on whether you loved or hated the thing.)
Yes, yes, I'm coming to the point.
My question to everyone is this: should reviewers pretend not to have a bias, or is it more helpful if we own up to it? If I mention the author's tendancy to make female characters interchangeable put me off the book but that otherwise it was a well-crafted story, does this put things in perspective for people who aren't bothered by the same things I am? Or does it just sound like a cop-out?