Twilight Princess - about 10-20 hours of filler killed the game for me. Throw in the huge, but completely barren open-world. The uninteresting characters that I didn't care about. The brown & bloom graphics style. I was very disappointed.
Super Paper Mario - Way too much pointless, uninteresting, throw-away dialogue.
GTA IV - Even though I finished it, I was bored almost every minute. Felt like such a huge step back from San Andreas to me.
Half-Life 2 - Put about 5 hours in, and it just seemed like a boring FPS, with barely any dialogue/story/characters, in a large empty world with repetitive enemies. I later went back and beat it, and felt the same way.
Super Mario Galaxy- game was great, but I got bored of the whole mini-planet levels. I prefer Super Mario 64 style large open world levels, or even Super Mario 3D Land style levels.
Fallout New Vegas - Ran into too many glitches, and felt the game was just sloppy all-around from the writing & story, to the world design. Also, the constant load screens were too much for me to take.