Let's see what comes to my mind from the top of my head... I mean, there are plenty of games I never finished.
Mass Effect 1 and 2, never got far because the gameplay is just so fucking bad and the "choices" are so bland it kills everything that was ever interesting in the setup.
Dragon Age 1 and 2, again, shitty gameplay, completely random combat (and way too much of it) and stupid choices.
Crysis, once the aliens came, I lost interest.
Grand Theft Auto 4, as much fun as it is at times, the realism really gets in the way of the fun most of the time, and thus I never finished it. Tried several times too, but just couldn't do it.
Fallout New Vegas, got actually pretty far, but the bugs and the shitty oblivion engine just got the better of me.
Assassin's Greed, too repetitive, enough said. After seeing the cities once, revisiting them felt as exciting as a pile of rocks.
And countless others, for various reasons. Some I can explain, others I can't.