I'm not a strict reader, I don't care if the book isn't what I expect it to be. So if I'm hoping for a book to be funny and it's actually tragic, I don't mind, as long as it makes sense. It takes a lot for me to be disappointed by a book, but the chances increase when that book is part of a long series. Series that I actually love, mind you!
What really disappoints me is when a character acts in a manner that is so different than anything he/she ever did in the series/book. So if a strong, manly, smart guy acts like a complete wimp all of the sudden, that might make me lose my focus. It's not about doing the logical thing. Even if I want to roll my eyes when a character does something that to me seems stupid, I realize that it makes perfect sense for said character. And to be honest, I'm not one to judge, I do stupid things all the time, like all people do. So it's ok for a character to make a mistake. But have a complete transformation and become someone else? That's something I don't understand.
A book also lets me down when the focus changes too much. If the real main characters of the book aren't the same as the one's the blurb said they would be, that is so disappointing for me. It pisses me off when I'm supposed to be reading the story of a certain character, but instead the subplots take charge one too many times, making the main character actually become second character in his own story.
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.
What book disappointed you?