I do mostly positive reviews for my own website, with a few exceptions, basically because I tend to spend money on and enjoy writing about books that I actually like. Writing here, I tend not to go on and on about bad books, but I do sometimes write a lot about books I've particularly enjoyed (usually non-fiction, like this, that and the other). However, I've also done a few reviews for Infinity Plus, most of which have been negative, basically because I owe it to the editor to at least be honest once he has sent me the book, especially since I haven't paid for it and it costs him money to send them to me.
What has tended to surprise me when I google the books I don't like is that most reviews seem to be very positive. I think this basically indicates that most people prefer to write about books they liked than about books they didn't. No doubt there is also a bit of a phenomenon of authors asking their friends to write nice things about them. Night Travels of the Elven Vampire was an unusual case.
I feel that the reviewer has a duty to be honest about his or her feelings. If you are able to write about the books you don't like, it makes your opinion of the books you do like more valuable. I think.