Skyrim: It wasn't that I didn't enjoy my time playing it, it's just that I spent so much time playing all the side quests, that I got bored before I even got through half of the main story. Eventually I forced it upon myself to do so, but honestly I felt good about it.
The Witcher: The combat, oh god, the combat...why does he hold a sword like that? He looks silly. I laughed out loud the first time I saw it. The combat was boring and simplistic on top of that, so I felt no engagement to keep playing the game. But I did anyway, since I'm a completionist, and must play a game series from beginning to end if there's any connection between the games story. (Games like final Fantasy obviously don't count...I couldn't even imagine playing all of those it the stories did directly connect)
Serious Sam (pretty much all of them): I refuse to play any game on its normal difficulty, and sometimes, if available, I won't even choose Hard if there's something above it. I did this with Serious Sam, and I was an idiot. I don't care who you are, you can only be a loser for so long before you just give up entirely. If it wasn't for the quicksave, I probably wouldn't have bothered ever beating any of the games.