UPDATE 16.Oct 2014: With the new Yosemite Beta Version 6 this bug is gone it seems. The screen goes to black from time to time though for 1-2 secs but recovers without me doing anything. I still have to install the final Yosemite bits...
UPDATE 20.Nov 2014: I guess, this had nothing to do with Yosemite. Also Beta Version 6 crashed later and it kept coming more often each day. Since I am still in my 3-year warranty (though only 2 months left) I sent it in for repair and since then (10 days) I have not had any crash so far.
This bug is - by far - the most annoying one of Yosemite (I still hope, it is actually Yosemite and not my hardware ... right now this is difficult to distinguish) My current setup is a MacBook Pro 15", Late 2011 and a Thunderbolt 27" Monitor.
Whats the problem?
Every 2-4 hours (it seems to be connected to the running programs) I get nasty graphics issues (horizontal and vertical lines, sometimes in green/blue etc) that make my OS X unusable. A couple of seconds later the whole machine crashes (monitor goes grey and thats it)...
How I "fixed" it
One thing I found that "helps" is to reset the graphics like this: open or close the laptop lid (sometimes do both). If you are fast enough, this resets the graphics and you can continue working without a crash.
I really hope, Apple is fixing this - and thats it not my hardware ;)