Oh, so many...
PRINCE OF PERSIA: WARRIOR WITHIN - After loving Sands of Time, I immediately bought the sequel. The hardcore metal music during the combat, a terribly confusing map and the destruction of the Prince as a character turned the game into turd within two hours of playtime. And, as Yahtzee brilliantly said, the Prince apparently let his mom cut his hair. "Emo Within" indeed.
NEVERWINTER NIGHTS 2 - My first attempt at playing a game of the series. But the annoying D&D based gameplay and the hideous turn-based combat destroyed the game for me. Not to mention the engine needed to be optimized.
MEDAL OF HONOR: AIRBORNE - It's still the first game, but now with thirty seconds of parachuting in the beggining of each mission, and now you can choose what objectives to do first. "See, it's not linear anymore!", EA Games says with a smile. Linear or not, dear EA, destroying AA guns has already gotten old.
FLIGHT SIMULATOR X - It's only fun with all the options in the max and high resolution. You see the problem right there.
SWAT 4 - Simply giving me a bunch of maps to kill perpetrators in is not enough, Sierra. An actual story would have been nice.