having the same problem on a 13" MBP running 10.9.4, 16GB RAM
Really don't believe this is a hardware problem. Usually happens waking from sleep, never when i am using. Originally thought it was some new software, but have stopped using it and still happening. Only fix is to power off by holding power button.
same **** Debug info for *possible* hang in MAIN graphics engine **** in the logs coincide with the freeze. Tried force-quitting every app.. still no good.
Just saw folks replying to this thread about similar issue in Mavericks... I want to mention that I have the same issue and it started appearing in Mavericks as well. Therefore I have created new thread in Mavericks section with all details I have found about it. Hopefully we'll find a solution...