The entire halo series is probably the biggest letdown to me. Bungie just seemed to steal ideas from better shooters, dumb them down, and throw them on a game with a sub par sci-fi plot and frankly a terrible campaign. Well, not terrible, but if a game is getting loads of excellent reviews and has its' OWN FUCKING SODA it should be the greatest thing in the world.
as for other games, I'd say warhawk was an disappointment to me. Maybe it's just because I'm terrible at that game, but ever since I got call of duty 4 every other multiplayer game seems lacking.
Everyone seems to have a problem with a oblivion, but I really love that game, even for it's respective weaknesses. Morrowind as well, though disappointed in some ways. I really enjoyed each game's sense of freedom, and the depth of cyrodil and morrowind are both incredible. But each game has something it's lacking. For morrowind, I was at first turned off by the fact that the main quest didn't really get going until about 10 hours into the game. I s'pose that's fine, seeing as the main point of the game is to do whatever you want regardless of the story, but for some gamers that can be a turn off. I also think that there's more realism in oblivion than morrowind. For example, the fact that the theives guild and dark brotherhood are accessible to everyone in morrowind seems kind of strange. To think that congregations of theives and assassins would be in plain sight of the public just seems funny. But the leveling system in oblivion is just stupid. At first, everything just seemed too easy to me, so I cranked up the difficulty a bit, and then I could barely kill the fucking wolves and imps. frusterating.