Complete system hang, graphics-card related ("IntelAccelerator: bad busy count (255,1)")
This is related to graphics card switching. As @nneonneo said, disabling "Automatic graphics switching" is one option but the better option is to use "gfxCardStatus" application and make Yosemite stick to integrated (Intel) card which will prevent auto graphics switching and won't drain your battery and run hot as it would if you did the "Automatic graphics switching" way (because it sticks to discrete card).

BTW: I can reproduce the problem on my MBPR Jun 2012. Go to this website in Safari, which triggers the automatic graphic switching. Refresh the page a few times and you will crash Yosemite. It seems to happen when it switches to discrete and back to integrated in quick succession.

Edit:
Using that website, it won't crash as often as before. Because it was a WebGL component causing the switch and one of the updates did something related to 
WebGL.