The biggest disappointments for me were Brothers in Arms: Hell's Highway, EVERY CoD game after the fourth game, (you'd think I'd learn, eh?) Force Unleashed (I & II) and - to an extent - Halo: Reach.
I got REALLY hyped for BiA:HH, for some reason. I don't know, I just imagined that the multiplayer would be a godsend with intricate tactics and intelligent firefights, but it was laggy and dull.
Modern Warfare sequels... they're all the same game with different, shinier weapons and WORSE maps. What's with that?
I don't know how, but the Force Unleashed games were utter crap. How the freakin' hell do you mess up the premise of "godlike powers of lightning and telekinesis, laser swords, KILL EVERYTHING"? I think Yahtzee described it best as too short and oddly repetitive.
Halo: Reach? Ugh, that was a huge disappointment for me. It's not a bad game, but I've been a huge Halo fan for years and I have expectations. The story was lackluster, nothing more than a military campaign. The atmosphere should be tense, what if the whole aliens burning the planet around me, but I just couldn't care about that until the end of the game. The level design for the multiplayer was weak, there weren't really any innovative new weapons, and... I'll just cut to the chase. Halo was slowly turning into a Call of Duty clone. That's why I'm excited for Halo 4, I guess, a clean slate to get back to the roots of exploring alien wonders.