For Western games I would have to say Dead Island, there was so much hype built up by their trailers, and then the game itself was a mediocre quest grind. I felt like I was playing some grindy zombie MMO, the story was just underwhelming, and the characterization was empty and atrocious. And the escort missions, damn those escort missions combined with terrible terrible AI.

For Japanese games I would say Tokitowa (Time and Eternity) was the biggest disappointment. It was touted as an epic HD animation game, a breakthrough for the RPG genre. The animation quality turned out on the level of doujin, the combat mechanics were repetitive, and the premise ridiculous. I know it wasn't meant to be overly serious, and they were going for a "so bad it's good" corny story, but this was so bad it's just bad... The only saving quality the game had was some absolutely hilarious dialogue at times.

Anyway these are just games I had expectations for to be good. I know there were plenty of awful games that came out (like Aliens or PSABR) but I never had expectations for them and thus I couldn't be disappointed when they ended up precisely as expected.

I would also have to say Other M, despite me being the game's erstwhile defender against those who see it as the greatest sin conceived by mankind, it certainly did not live up to its own initial hype, and may have put Metroid on an indefinite hiatus (or at least that's my fear)

The ending to Mass Effect 3.

And calling it a "disappointment" is extremely generous.

For me personally, Mass Effect 3. I never played a Mass Effect game up until 3, and considering all the hype for it, I was expecting a whole lot more than I got out of it. To this day, I still don't understand how anyone gets so excited over what essentially amounts to a choice driven segmented cover based third person shooter (especially considering that all those choices made during the game really didn't mean crap during the ending), but hey, millions of people buy Call of Duty each year and I don't understand that either.

For a fanbase as a whole, Metroid: Other M. I still don't think Other M is a "bad" game, and if it wasn't part of the Metroid franchise I might even call it good. However, the hype for that game was sky high, and everything that the game was supposed to do, such as explaining Samus' complex relationship with Adam and delving into Samus more as a character, was botched horribly. Ironically, the one part that didn't get a whole lot of focus, which was the combat, ended up being the game's saving grace (or, at least, saving it from complete and utter disaster).

Dishonorable Mentions would be Dead Island and Sonic '06.

Bayonetta.

It's just an overrated copycat of Devil May Cry/God of War except you play as a prostitute/hooker or something like that.

I don't understand what's the draw in. Shit game and ppl are hyped for sequel. lol

and IT'S VERY FUDGING EASY!

GTA4. Bought it with my ps3 in 2008 and still havent finished it.

CoD black ops. no this isnt any CoD hate. i love MW2 and i played the thing like crazy. at the end, our CoD MW2 stats had over 1 month played (in game) and prestige 10 reached. i think over 75 thousand kills if i remember correctly.
we got on the hype train for black ops, we didnt do any research about the game. me and my brothers bought it day one and wow were we all disappointed. the only thing good about it was split screen online

Super Paper Mario and Other M.

probably GTA4. I played about half of the game and could'nt finish it. It was such a bore and a step back from the previous games. I love rockstar and this was one of the few times I felt they let me down.