RAGE was a huge disappointment, but there were quite a few of those over the last year. I felt like an idiot for buying it, because the only reason was basically the reputation of the developer. So I consider that more of a personal failure than that of the developers. I wasn't that impressed by the previews. Gut feeling, I'm sorry, I won't ignore you ever again.
Brink should've been one of the best games of the year. Instead it was just a poorly designed mess, and easily the worst game by Splash Damage yet.
I bought Crysis 2 because I loved the multiplayer demo, only to find that they completely fucked it up by hiring Gamespy of all providers to handle their accounts. Predictable disaster? Yes. And much worse for all the bragging they did leading up to release.
I didn't actually buy LA Noire, it was a gift, but I did find it very underwhelming. Who cares about facial features when all the women in the game look like they were born and bred in the Uncanny Valley?
Then you had Deus Ex:HR. It was disappointing on three counts: Terrible writing, the fact that it could've been a great game, and that people seemed to buy the hype more than the game itself.
BF3 was actually one of my biggest surprises. I guess I had much the opposite experience of AHA-Lambda. I was pretty disappointed in all the start-up troubles, the battlelog and network features felt half-finished, if that, and I'm pretty sure they rushed it way too hard, but I felt like it was a huge step forward from the mediocrity that was BC2, even if the bragging, once again, makes it's flaws all the more glaring.
My biggest disappointment of all time is easily Mass Effect 2. If I had realized that they were gonna completely reboot the series as a generic Ubisoft third person shooter with a laughable plot, there's no way I would've bothered. Most of my friends who got it didn't even finish it. I finished it, only to regret I did. I'm still trying to forget, so I can enjoy the original for what it was.