I've left games because as a player I could not trust the DM. In the first case, he'd asked for 1st level characters, and then gave us an adventure in which our only chance for survival was to stand back and let the 5th level NPCs do everything. Gee, that was fun. Second case, the DM blatently favored another player, then went and changed my PCs action because he thought my PC would get killed (my POV: my character, my choice to take the risk), and to me that was the last straw. Third one was an on-line play-by-post game with a DM so overcontrolling that I was signing in just to find out what my character had done. I might as well have been reading a fantasy novel (not that there's anything wrong with that) as playing--and since I felt like he had 95% control over my character's actions, it wasn't that much of a wrench to say, "why don't you just take the character over completely". Of course, over 20 years of playing, I guess that's not a horrible record.
The games that I still play in, I trust my DMs to create an adventure that I'm going to enjoy (and in which I am the one who controls my character).
The games I DM, I feel that my players do put in a lot of effort in terms of roleplay, but I have put some restrictions in place for character creation so that their characters do fit into my homebrew world. Of course, I also tend to create adventures that hinge as much on whether the character can stay true to stated ideals as on whether the characters can overcome external foes, and even straight 18s (not that I would allow that) won't help achieve the internal victory.