I never let myself get disappointed with a game, particulary because I try going with a clean state of mind and no hype when playing.
That being said, my biggest disappointments are:
Bioshock: A FPS game that is, by no means, the quality of System Shock 2. Interesting story and setting (good atmosphere too) doesn't make this dull shooter much better. I've never understood the appeal of a character that five minutes into the game is inyecting himself with some syringes (And YES I KNOW WHY HE DOES THAT LATER IN THE GAME), that suddenly becomes too OP and indestructible. The prepotence of a certain character didn't help in my opinion, though everyone else seems to love that char. Not a bad game, but certainly not the Opus Magnum I've heard. I actually enjoyed Bioshock 2 inmensely more because everyone hated on that game, while I think the quality is superior.
El Shaddai: Ascension of the Metatron. A shame of a game. I actually enjoy the artstyle, and the Biblical-based story was an amazing touch. Everything else fals appart. I'm no fan of hack n' slash games, and certainly this game is barebones with that system. Hiting the same enemy five minutes for him to die (Not to mention having to purify your weapon every two minutes) isn't my idea of fun. Still haven't beaten this game, and it is probably one of the few games I own I've never ever beaten.
God of War 3: One of Sony's most iconic franchises, and the first GoW I play. Didn't like it one bit. Impressive graphics and set pieces, but again, a simpleton hack n' slash style (game isn't as easy as El Shaddai though) and stiff movement with Kratos that leads to much annoying deaths. The story looks nice, though, so I may get to beat this game one day.
Halo: Reach. Almost everyone agrees that Halo: Reach is a masterpiece. It is not. It is the most derivative, dull FPS from the Halo franchise. The story was "good", but it was presented like crap to the player, no connection with the characters (seriously, they're cookie_cutter characters!), big plot holes, not a good soundtrack like in other games and cheap difficulty...what's here to love?
For the record, I never played it online, so my experience wasn't complete. But still, campaign is atrocious, and made me lost all love that was left for the Halo franchise.
The Fire Emblem DS game. A remake of Marth's adventures turned into a putrid artstyle, bad animation, no connection with the characters whatsoever (I mean, I didn't play the original, but whenever someone died no one else cared?) and easy for Fire Emblem standards (though you could pretty much put Lunatic at the beginning and that's pretty lunatic). Seriously what the heck happened here. I just beat two chapters and never looked back again.
Dishonored: I'll never understand myself with this one. This game has all the elements of a great game, yet it bore me to tears. I've tried playing it seven times, only to give up all those. Definitively not a game for me, though I loved Thief and Deus Ex (all games of the franchise), so this comes as a surprise to me. Certainly a disappointment, but not the game's fault this time.
Spelunky: Dunno why this game got so much recognition, but I didn't like it one bit. The fact that a monster comes and kill you if you take too much time kills the enjoyement even more. WTF.
And the music was certainly annoying.
Viva Piñata: I always had high hopes of loving this game. I didn't. Again, not the game's fault, but certainly a game that is not for me. I did love Kameo, though, so at least there's a Rare game I've managed to enjoy in the seventh gen.
