Every time I read an old "classic" novel, I'm surprised that it isn't at all what I expected. Off the top of my head, Frankenstein, Dracula, and Jekyll and Hyde were not what I anticipated. Frankenstein I knew was the doctor, but I had no idea that the story was all about the monster's search for--what? Humanity? Companionship? Accountability? In Dracula I was under the impression that they spent the bulk of the novel trying to figure out what he was. They knew right away, and spent the rest of the book tracking him down. Jekyll and Hyde was unexpected because half of the book is his written confession. Part of my misconception is due to the musical, which adds women and all sorts of subplots, because there actually isn't much plot to go on.
I guess my ideas of the books are formed by other things--TV, movies, musicals, popular cultural conception. It's just weird how many times I read a book and say "That wasn't what I expected at all!" Anything that took you buy surprise?