I'm going to have to say FFXIII (although note that I did not play many games this year). Not because I had high hopes, as my expectations were very low. But because while playing the game itself, it got my hopes up, and then dashed them horribly.

Through the first 8 chapters, the linearity and poor design of some aspects (upgrading system, anyone?) were grating, but I was pleasantly surprised by an engaging and interesting battle system and in-depth character development. Then the end of chapter 9 came, which was stupid. And then I had to watch the last 4 chapters of the most horrifyingly terrible storytelling in any game I've ever played. Chapters 10 and 13 were almost self-parodying with how bad they were. Very, very disappointing.