I like complaining as much as anyone, but I need to preface this with my overall impression of this generation. I think this has been one of the best generations ever, probably top 2 in my mind. I'm surprised by the hate for Fable 2, me and my friends loved that game, not perfect by any means but still very good.
Okay biggest disappointment, I guess I will join the bandwagon and say the Wii. I got one the first week but it barely gets turned on. As far as games, GTA4, RE5, Mirror's Edge, and Assassin's Creed.
GTA4 was a tough one for me to put in but after playing the heck out of the GTA3's I couldn't sink more time into the same game. I'm probably alone in this one but after playing Saints Row the controls for GTA4 were painfully sluggish.
RE5 - Still an ammo management simulation game. I still can't walk and swing my knife? How do I shoot, press hold one button and press another? Why? Controls killed the fun for me.
Mirror's Edge and Assassin's Creed - As everyone said, great concepts but poor execution. Mirror's Edge trial and error ended it for me, and assassin's creed became repetitive quickly.
The games I listed I consider good games, but they had much more potential.