The C&C3 campaigns, for some reason the next mission wouldn't load, and I never bothered since.
GTA4 for overal repetivity, and the uber annoying friend system.
Assassin's Creed for, again, overal repetivity.
Same for Far Cry 2.
Starcraft (plus Brood War), for some reason I couldn't be bothered.
The Settlers Rise of an Empire campaign, I enjoyed random skirmishes for a while (it really is a good game, just too simple for a Settlers game) but the campaign took too long to take off.
Ninja Gaiden 2, seeing my little brother fight the last boss freaked me out enough to not touch the game again

Overlord, my brother's game but I couldn't be bothered with it.
Splinter Cell Double Agent, same as above.
Probably some more, can't remember though.