Four jump to mind:
The Elder Scrolls IV: Oblivion - This game had me so excited, because I had just finished playing Fallout 3, which is one of my favorite games of all time. After a few hours I realized that the combat sucked, the voice acting sucked, the plot sucked, the world sucked, the animations sucked, and the main quest sucked. On the upside, the guilds and side-quests were quite imaginative and fun. Overall, I hated it, and since I got all of the ridiculously easy achievements, I'm happy to say that I'll never play it again in my life.
Red Faction: Guerrilla - After seeing trailers for this, it looked like a great strategic game with huge places to blow up. What I found was a disappointing game with a very empty open world that should have had huge cities to destroy, a pathetic health bar, retarded friendly AI, weapons that failed to impress (Other than the ridiculously awesome Nanorifle), a worthless ammo carrying capacity, and a paper thin plot.
Half-Life 2 - After all the hype about this game, I couldn't imagine not liking it. I don't know if it would be fair to say that I "Hate" this game. In concept, it should be good. I can ignore complaining about the plot since I never played Half-Life 1 and really can't understand it anyways. My main problem was that the level design was awful. I would find myself stumbling around for half an hour looking for my destination before finding a staircase tucked away in a barely noticeable corner somewhere. Also, and this is something I think even the lovers of this game can agree on, the driving controls are probably the worst I've ever seen in a video game. Who was the genius who decided to map everything to the left stick? It's a mess! On the upside, Episode 1 and Episode 2 were much better with much more straightforward level design.
Super Paper Mario - Now, the Paper Mario series is one of my favorite gaming series' out there. I loved Paper Mario, and The Thousand Year Door is one of my favorite games of all time. I heard that one was coming out for Wii, and when my friend got it, I jumped at the chance to play it. What I found was a bland Mario platformer abusing the Paper Mario name to sell more games that sucked all of the fun and creativity out of the series.