This is certainly an interesting question, and it's a situation I've rarely found myself in, mostly because I make it a point not to read books I think I won't like. The only exceptions are a few YA novels I read out of morbid curiosity and they all turned out to be terrible. Anna Dressed in Blood is the only book I can think of where I went in thinking I wouldn't like it and ended up loving it.
Before reading it I was pretty sure I was going to end up being Debbie Downer once again. You may wonder why I thought this with rave reviews from nearly all of my friends, an awesome cover, and an intriguing summary. Well, I have a confession to make. The summary didn't impress me, and I of all people know you can't judge a book by its cover, otherwise Fallen would have been the greatest book ever written (excuse me while I laugh hysterically) and Dairy Queen would have been one of the worst. I'm also not a fan of ghost/human romances, and I'm always wary of books with male protagonists written by female authors due to some past negative experiences.
I was pretty sure I was never going to read Anna Dressed in Blood but I couldn't completely write it off because of all those glowing reviews, so I kept it my To Read list. I might never have gotten around to it if it had not been for Goodreads Reading Challenge. I desperately wanted to make my goal and I was about nine books behind in December 2012, which meant I needed some shorter books to read. Anna Dressed in Blood fit the bill, and it turned out to be one of my favorites of the nine books I read that month.