Was actually just thinking about this the other night as I went through my installed games list and uninstalled some games I had started but never finished.
The reason that they were uninstalled was I had felt something had been missing and I just didn't feel the urge to finish them e.g. too generic, crazy difficulty levels etc.
The other reason I normally quit on a game is I had got distracted from playing and then found it too hard to get back into them usually a story based game (RPG).
Actually bring me back to the Games you have but not yet finished thread from a while back and why they are unfinished!