I have to say that Portal 2 was a bit of a disappointment, I probably expected it to be longer and not so much emphasis on the online.
Same goes for Resident Evil 5 and Lost Planet 2, which I already talked about.
Gears Of War, because it was monotonous, the gameplay was a bit boring, I felt apathic throughout the whole game.
Now, for Retro disappointment, I'd go for Fear Effect. I played it and I raged. The controls are unresponsive, the whole game-set is a bit confusing. I always heard good stuff about this game, and then my dreams got shattered.
There are a lot more games, that I've been disappointed with and that's only what I remember for now.