Lemesee...
Arcanum: Of Steamwork and Magic Obscura, recently - I just got bored with the empty-feeling environments plus I never really understood the combat system and never really got much involved in the story. So I stopped some 10 hours or so in. A shame, really, as I was looking forward for some good cRPG...
The Last Remnant - my first shot at JRPGs, the further I got, the more I felt that my lack of understanding of the game mechanics lead me to have really bad character builds, got into an impossible fight with some boss (pretty far into the game, I think), tried quite a lot of times and then stopped playing.
Far Cry 2 - never really got me interested, eventually quit to play something more fun.
Magicka - but that´s a specific case of me just waiting for a friend to get his PC fixed so we can play it together...
Then there´s this thing when I sometimes stop playing a game for quite a long time without any reason whatsoever but then pick it up again where I left and finish it a couple of months later. This happened to me with Mass Effect, BioShock, Dragon Age: Origins and quite a couple of games, actually. Also SpaceChem but there it´s the difficulty of the new levels that turns me off every now and then only for me to return much later to give it another try...