There aren't a lot of things that can let me down, but when that happens, I get sort of pulled out of the story and I feel like I'm looking at the words, instead of "seeing" them come to life. Sounds weird, but it is what I feel.
Anyway, a book that has disappointed me was Dead in the Family by Charlaine Harris. The reason is that for some reason, in this installment of the series, Eric is a shadow of himself. He's not the sexy, dangerous, powerful, strong, sarcastic, fun, entertaining, manly vampire we've known for 9 novels. I don't remember anything happening that could make him change that way. It's actually the only reason I didn't finish the book the first time I tried reading it and I'm really wary of trying to read it again. I liked the old Eric, with or without his memory. But the Eric we have in DITF isn't the one I like.
What book disappointed you?