Extreme difficulty, I just have a certain threshold that once its passed I cant stomach it anymore. Other than that, not much, Sometimes in the case of RPG's it can be realizing some game element I missed before, like for example enemies scaling with your level in an RPG, or something else that is just too much of a system shock from what I had originally thought to keep playing.
Again though, the only solid one is ball crushing difficulty, since I either buy games or rent them from gamefly, either way I try to finish them before I send them back just for the closure.