I started to reply to this thinking only of one game, and then remembered several others. Fable 3 is one that really broke my heart. I loved Fable and Fable 2 so much, I still replay both of them all the time. And then Fable 3 came out, got me all excited, and turned out to be a soulless husk of the previous games with everything good sucked out and replaced by DLC, because they didn't get enough of my money when I bought the stupid game. ARRG.
Spore was also a huge disappointment. I was amped up about that game for absolute ages before it came out, being a huge fan of the Sims (I'm female, sue me) and young enough to still look up to EA in naive adoration, thinking it was some kind of Santa Claus, not realizing the abusive drunk husband it really was, beating the crap out of you then promising the world to make it all better only to crap on you again. I still play the creature level sometimes, but then I reach Tribal stage and there's just no point...
I can't believe no one has mentioned Bioshock 2 yet. Jesus what a kick in the crotch that was. Perhaps it was exacerbated in my case because I'd just finished playing through the original for the first time when it came out. What a spectacular example of how to take all the exact same elements of a thing and put them back together completely wrong.
Fallout New Vegas. Auuuuuuuugh. I expected so much...
Elder Scrolls Oblivion disappointed me too actually. Wasn't really the game's fault though I think. Everyone seems to think it was pretty awesome, I'm in the minority for disliking it. I only played it because my friends kept harping on about it and how if I liked Fallout so much I'd love it, on and on and on until I gave in, by this point expecting the messiah of all gaming to leap out of my monitor and punch me in the face while simultaneously making me orgasm. Instead I got...Eurhg. Okay. So I come out of the dungeon at the beginning of the road. I don't really pay attention to the road at first, wandering off to enjoy the scenery and pick herbs and, about five minutes later, be attacked and hideously murdered by wolves. Okay, I thought as I went back to my auto save outside the starting dungeon. That didn't work. Maybe I should stick to the road? So I start jogging down the path. I get just far enough to think, 'jegus this road is long, are they really going to make me hoof it all the way without so much as a mini quest to break things up?' when I am again set upon and devoured by a pack of wolves. I'm a little frustrated at this point, but I persevere, going back to my save and trying again, and again, and again. It is at this point I realize I am being SPAWN CAMPED BY WOLVES (and once a gosh darn BEAR). Clearly, I decide, this game is not meant to be played on foot. I need a horse. So I find the nearest horse store on my compass, miraculously make it there without being murdered by fricking wolves, and discover I can't afford a horse, but it is possible to steal one. So I do, and ride off into the sunset. At last, I think, I can begin to really play the game! And you know, this scenery really is gorgeous when my face isn't being gnawed on by predators. I might actually like this game! Warmed by the glow of baseless optimism and the satisfaction of leaving a certain persistent wolf pack in the dust, I carry on down the road and spot a couple of riders heading my way. Maybe they're traders, I think! I can sell some of this junk from the dungeon and buy some better equipment. Ooh, maybe they'll have a quest for me! So I dismount and jog up to them, cheerful as can be, at which point I discover they are guards sent to apprehend a horse thief... and they murder me.
RAGE QUIT. NEVER AGAIN.