Four games come to mind from this gen. All were below expectations for their franchise in at least one aspect. In order:

1. Final Fantasy XIII- took everything that makes Final Fantasy great and threw them out the window transforming it into purely linear game with an RPG battle system.
2. Dragon Age 2: good story and characters but horrible combat that relied on endless waves of enemies for difficulty thereby removing all strategy that was present in Origins. The re-use of assets and level design was also unforgivable.
3. GTA IV- after San Andreas, this was a major step back for the series; less vehicles, smaller sandbox and annoying phone calls from "friends".
4. Duke Nukem Forever- all that time waiting for a game that feels like it should have released in 2004 (if not earlier).

Diablo 3 by a country mile
Probably Duke Nukem Forever after that

Final Fantasy 13 for me.

Going into this gen FF was one of my favorite franchises....that all ended after 13.

wow a lot of people didnt like GTA4 It is one of my favorites this gen. It was nothing like previous GTA's but it was still a good game at the time. I feel like a lot of the hate it gets only started some time after the initial release.

Super Mario Galaxy

I got suckered in to buying the game, because couple of my friends acted like it was like the second coming. Played it and it was just another platformer, boring.

It is not a copy, it isn't overrated, but it is indeed fucking easy.

crackdown 2
crackdown 1 was not a great game but with a lot of potential and more missed opportunities,but it was alright.

After so many years I thought crackdown2 will be really great but this games was just a rippoff,an unmotivated low budget dlc not worth 5 dollars .They only digged some hole in the ground and the mountains-that's the only difference to part 1.
as this game was not released officially in germany i paid 70euro(90dollars)for this piece of sh....

dragon age:because it was so ugly.really dissapointing for a bioware game as they usually have good graphics

Prototype my disappointment turned into hatred when the game pissed me off.

Mine is definitely The Last Guardian. It felt incomplete, didn;t even have a release date.

Bayonetta- the amount of hype and praise people were giving this game was nearly on par with God of War in 05, when I finally got the game for $15 thinking it was a steal only to find out that the game consists of 25% hack n' slash/25% vehicle riding/and 50% sexual innuendos...

Dragon Age II-I just don't know how Bioware managaed to not only take a step back but fell in the damn well behind it...onlt one city and rehashed dungeons!? GAWD DAMN!!! to goty to just plain shit!!!

Tales of Symphonia: Dawn of a new world-...if I wanted to play pokemon I would've played my DS(and they're freaking useless on top of that)

I can't remember being disappointed by anything this gen.

Life's guud :D