Blue Dragon was a dissapointment because it was bland and poor introduction for a studio that was suppossed to be the new Square. Yeah it moved 360s in Japan, who cares, it's still dead there.
DMC4 was a dissapintment for some in that its story was an awkward juggling of the mess that lead up to DMC2 and the game itself was really only half a game simply mirrored to create the later half. Gameplay is phenomenal as always, but the unique mechanics of the game (dice rolling) really just made progression awkward and the huge cut scenes at the beggining just felt like a waste in that you weren't allowed to play them. Many people I know personally never finished the game, they just got bored. There is no arguing the game had no real direction and was basically treading water.
People will tell you DMC4 was awesome, but there are people that will also tell you that LAIR is awesome. Now I'm not saying DMC4 is anywhere near the level of LAIR mind you, but when online there is no shortage of people willing to defend or attack something. But in the end just remember that graphics go a long way in whether people see a game as being good or mediocre this generation.
Assassin's Creed was a dissapointment in the eyes of many fans but not in terms of reviews or sales. It like Army of Two are prime examples of games appealing to the lowest common denominators.
MKWII was a dissapointment only in Game Reviews, other than that its been a success in every other measure and oddly enough the most fun Mario Kart I've ever played.
Okami for the Wii was a dissapointment too, at the very least in sales. Beyond that there's no reason they couldn't have included the ending or added an alternate control scheme for the Classic Controller to make this the definative version like RE4 for the Wii was. In the end they just made a compromise of a port.
Now for major dissapointments, let's try on Nights for the Wii, there's a dissapointment.
Haze, Free Radical, I thought you different from the other FPS makers... sigh...
Turok, sucked, the only real crime is that it sold as many copies as it did. The franchise has been dead since Turok 3, just let it die people.
Orange Box for the PS3 sucked, but then again many ports for the PS3 have at least one problem or another.
Unreal Tournament 3, I thought this was suppossed to be one of the greatest FPS franchises?
Halo 3, talk about the most overhyped game of all history.
Heavenly Sword... Five hours long, give me a break, a game hasn't been this misrepresented since the original Fable.