At least there will be Black Mesa Source
Lets see my most disappointing three games.
Deus Ex: Invisible War
Not that this game is horrible (The story was alright, and it's gameplay is similar to Mass Effect 2) but that it was supposed to be a sequel to Deus Ex is where it made me hate it. It felt like they removed the freedom of choice, the deep cyberpunk universe, and the fun/funny dialog.
Red Faction 2
I loved Red Faction, but Red Faction 2 wasn't anything like it. And I don't like things that are different just for the sake of being different (and completely a step back). The graphics were worse, the voice acting was not good, and you weren't even on mars. And most of all you couldn't blow walls up.
Left 4 Dead
The reason this is up here is because I like games that are made by Valve, the only game I can think of that I don't is Left 4 Dead (and Counter Strike, but I view that as a mod instead of another game). I'm not the biggest fan of Steam, but games by Valve are supposed to be fun. I don't like any of the characters, the weapons, the overdone zombie setting. And the fact that you rely on people that seem to be content on running around getting killed than helping their team. Ad don't get me started on the A.I. The only enjoyment I got was killing the survivors as an infected but that got boring after about 2 hours or so.

Oh com'on, that game is really good.
The problem is only that the game is to overrated by "gamers" (meaning casual whiny bitches who never touched a real game in life except the mainstream shit like KIngdom Hearts 2), but despite this, a good game.
You have to look for the details, for example how he gets dirtied quickly and cleanses himself up, the various interactions with Farrah ensuring lulz-potential, the enemies and swift animation,the riddles, the funny dia- and monologues, the surrounding theme and the awesome music. It's more the profound detail and exploration and less the basic gameplay like combat. But for a 2003 game it had quite the decent combat. WW patched everything up in gameplay, but is not as good as SoT if you ask me. But I love both as much.
Really true. But the good thing? You can mod RE4, improve the graphics(even more than on the PS2) and get the fixes right. For example, removing the terrible PS2 buttons and switching them to PC-Keys:
http://www.residentevilforums.net/mod-releases-mod-questions-f4/resident-evil-4-keyboard-replacement-keys-mod-t264.htm
It's alittle patient asked to get used to modding, but for RE4 it's a hell of worth it. I made a sound pack once with all Kratos sounds of GoW. That was awesome.

Thanks to paldepind for resurrecting this thread - it's been entertaining reading.
There have been few games that have truly made me regret buying them, but my top few have for the most part already been mentioned:
Tabula Rasa - much hyped, but in the end it was mostly borrowed MMO features and more of the same old grinding. Neat features, like skirmishes for control of outposts, just felt futile in the end because you knew it was just going to happen all over again.
Ultima 9 - I'm a bit of a completionist and will usually plow through even the most mediocre gameplay to try and find out where the plot is going. This was just so bad - both technically and gameplay-wise - that I just couldn't bring myself to finish it.
Final Fantasy XII - I really enjoyed FF7 and FF8, and had a decent time with FFX and FFX-II as well. I got about 30 hours into the game before I realised that I still wasn't sure what was going on, and I didn't care enough about any of the characters to want to find out.
Hoyle's Card Games 2008 (at least I think it was 2008) - the only game I've ever taken back to the store since Half-Life was released (in that case, because one of the store's employees had stolen all of the CD keys). I spent a day fighting installation and patching issues, just trying to get it to launch, and then it went downhill from there.

Cool thanks dude.

Far Cry: Enemies who can shoot you with pinpoint accuracy from quite a distance and a plot which goes completely bonkers. Ugh. The potential and the technology was awesome though. (Bought it for €5, sold it for €10)
Far Cry 2: I wanted to like it and I kinda did. For a few hours. But it quickly becomes boring with nothing much to do in this giant beautiful world with a cool bushfire-system and truely spooky nights. Really a shame Ubisoft made it so damn boring. Thankfully, Just Cause 2 is everything Far Cry 2 had to be and more!
Grand Theft Auto IV: Don't get me wrong, it's a brilliant game, but as a follow up to GTA San Andreas it just didn't cut the mustard for me. Liberty City is an impressive place to be, but all the fun and sense of exploration found in San Andreas was lacking. Thankfully, Just Cause 2 is a true spiritual successor to San Andreas (although the story isn't half as good as San Andreas' story)

Prince of Persia - Sands of Time (or rather, potentially great platformer type games, with shitty camera)
A game that I wanted to like, because it starts out good, but as it progresses the faults of the combat system starts to show, and the camera... I want to see every camera designer of all games with a similar system tied up and shot in the kneecaps, then left to bleed to death. So much wasted potential in several similar games, all completely destroyed by a horrendous camera. Never, and I mean FUCKING NEVER alternate between a controllable and fixed camera when in movement. Especially not when there's jumping involved. Never force the camera to point in a an opposite direction of where the player is headed. The frustration of such things has caused my fury to wrap around the reaction spectrum, starting from bashing the table in frustration, to calmly exiting the game, uninstall it (if a PC game), place the disc back in the cover, place it back on the shelf and do something else. It is almost absurd.

Oblivion
"Plastic" world, npcs are wooden, level system the most idiotic ever seen, monsters scale with your level, dungeons are all look-a-likes. But aside from these things I did play this a lot and there are some good quests in it. The assassin guild quests are pretty good.
Can't really remember anything as big of a failure as Oblivion (with so much missed potential).
Heroes of Might and Magic IV
Doesnt hold a candle to HOMM II or III, decent game tho. Expected much more.
Command and Conquer: Tiberian Sun
I was pretty hooked to Red Alert so I got this, but got extremely bored of it and didnt really play much.

Is it EA's fault if your computer sucks? And I don't get the argument that you are trying to prove with Dante's Inferno (which I heard wasn't even that great). And I want to try Dead Space so bad, it looks awesome!!
PS: I don't know if this is affecting your experience playing Dead Space and saying that it is a poor port, but I heard that if you take off V-sync the sluggish mouse thingy goes away. Which actually goes to prove your point that it is a crappy port, haha!

amd 3200 single core
4850
2gb of ddr1
game works flawlessly. you fail not the game :)

I have Dead Space for the PS3 and it is one of the few games that scared me witless. I couldn't even play sometimes. It is SO much better on the consoles! No bugs! Nothing.

I'm thinking of trying dead space on the console.. id love a good scare...
And i have to add one more to my prior list:
Brutal Legend F*** This game to hell... it was a lie, 20 minutes of kicking ass to awesome music and a very fun open world your forced into some RTS bull and never get to have fun in a dungeon or on your own again... this is the biggest waste of money i ever played in my life; never been so disappointed in a game. I don't even want to sell it because someone might see it and buy it thinking it could be fun, only to get mind raped and end up crying in the corner or kicking there puppy because of the rancid crap that is Brutal Legend. Id rather drink scorpion venom and run naked though a cactus farm then ever play it again.

Are you playing it with low specs? I found out that the game theoretacly does work flawlessly if you turn down every graphic option. JUst tried that out. Works better now.

Much to my surprise, my totally-non-gaming, cheapest-i-could-find dell laptop could run Mirror's Edge and it looked gorgeous and ran smoothly. Amazing art style.
1st half was one of my favourite games ever, second half was horrible. Weird.

Twat.