This list is somewhat easier to populate than the opposite, imo. I'll get it started and give it a go:

 1. Doom 3: As original as butter on toast and as deep as drunken philosophy. Boasted graphics that were well beyond anything the world had seen, releasing a demo called "Doom 3 CPU killer" on various forums. Well, guess what? The game was postponed (a great deal) and looked both bland and uninspired by the time it arrived.  The horror elements were more a gimick for sales than a real factor in gameplay, and the "peek'a boo" tricks grew old before they were born.

2. Auto Modellista: Cel shading would be all the gas, some people said a few years back. Lending from good old anime/manga style, it seemed like a cool ideam especially for those who had seen the geniousness that is Initial D. No driving physics, disappointing customization and some of modern days most anonymous sounds and soundtrack were what we got. No one expected GT-like features, but perhaps the easy going entertainment package you get in Ridge Racer games. They did not get that either.

3. Final Fantasy X-2: This is in no way a bad game per say, but it belongs in the disappointing category for me. The idea seemed nice enough, with the first true sequel in FF history, building on an already established and embraced story and world. This game had monstrous potential, but wrecked most of it by adding little to no content that unfamiliar from FFX. And the sheer fact that you can actually sing and dance in combat makes the hairs on the back of my neck stand to this day...

4. Betrayal in Antara: A horrid experience, not viewed on it's own, but when compared to it's acclaimed and extremely well made prequel; Betrayal at Krondor. Graphics were decent, and the movie sequences were actually cool, but the story was uninspired and tedious, and animation glitches were far more abundant than in the prequel (like the fact that you always swung your weapon THROUGH your opponent regardless of whether you hit or missed). The depth and customization of characters were also lacking, and none of the PC's seemed to have an interesting background.

5. Battlespire: In 1995, my friend and I barely slept for two months, and TES 2: Daggerfall was the to blame. The sheer size of the world (bigge than the entire UK!!) was breathtaking, and the story and sidestories were sublime. For it's time, it also had very good visuals and sound, and there was limitless potential in character variation and development.  Then came Battlespire, which was the direct opposite of Daggerfall; no depth, poor visuals for it's time, virtually no story whatsoever and a VERY confined game environment (ONE castle/tower throughout...). This is perhaps one of the most disappointing games in my life, and delivered little besides shudders and shaking heads.

Resident Evil 4.

As a big fan of resident evil,having played the 1,2 and 3,i got really dissapointed with this one. I loved the tension where zombies came outta nowhere and you continue running out of herbs/bullets, because resident evil is a survival horror. RE4 was just a action game. And the ending was terrible, but won't talk much to not unveil spoilers...

True Crime streets of LA

 I dont know why i ever bought this game, maybe the packaging looked cool, worst controls, just plain boring.

Crash Bash PS1.. I thought it was another platformer.. than I found out it's a party game AND that it's not made by Naughty Dog. It just plain SUCKED.

Metroid Prime Hunters - as much fun the online player was it was just a Halo for the DS. I love the online and I love the game don't get me wrong.. it's just NOT Metroid. At. All.

Odin Sphere: This game is the best looking game I've ever played and I wanted to enjoy it so badly but unfortunately the game design just sucked. They made this game hard just for the sake of being hard and eventually made me quit the game before I saw barely any of the great art.

 Hah, I own that game too! It's really really poor though... I take it True Crime: Streets of New York is of the same caliber? I've not yet had the "pleasure" of trying it.

Castlevania PS2= I was hoping for a 3d action title and got one except you could easily beat the bosses except the final one with a few shots of the right special weapon and the story sucked.

Mortal Kombat SNES= Somehow without the blood and most of the right fatalities this just didn't seem as appealing.

Street Fighter Champ. Ed. (Genesis)= The music wasn't right and neither were the graphics compared to the SNES.

Final Fantasy Tactics Advanced= Would have been better without the Judges.

Lost Planet - Story was horrible...

Star Trek Legazy - I can't put into words on how much I hate that game

Final Fantasy VII: Just after going from the super artistic style, great music and complete story of FFVI to the shallow story, plain graphics, midi music and clichéd characters of FFVII was a real letdown.

And that's the only 'disapointment' I can think ATM.

Onimusha- Short Short Short

Wario Ware Smooth Moves- The gamecube version was a slick party game at a budget price. The Wii version was boring, glitchy and focused on 1 player, so they charged $50.

Red Steel- I really didn't have a problem with the control. Just the fact that it was marketed to be a sword wielding game and that was very limited.

Halo 1 and 2- Yes good games but I have never enjoyed them and more than most FPS. Most people seem to think these games invented the genre and informed me that I would have some sort of game-gasm whenever I played them.