Throwing another into the mix (and going back a bit): Dirty Rotten Scoundrels. The trailers made it look like a bathroom humour fest, but it was much, much better than that. Plus with Michael Caine + Steve Martin, how can you lose?
And one that looked like a brainless teencom but was so much better: Easy A.
Agree with others about Galaxy Quest, How to Train Your Dragon, and Enchanted, all surpassing my expectations.