I've never quit because of another player, but definitely because of GMs. One incident, I've already gone into on these boards, so I won't bring that up again. The second involved this GM who had such high powered NPCs, one of the PCs was killed by (swallow what you're drinking now or you'll spit) a TROUT. He would also railroad the PCs into doing what he wanted them to do. It got to the point that I was playing with my dice more than paying attention to the game. 
