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on my late2012 iMac it shows a black window and the it tries to wipe over to the fullscreen, but in the middle of that wipe it breaks up with no message etc. :(
*edit* and the dock dissapears; restarting steam has no effect

are there any startup option in steam i have to insert? etc.?

Steam overlay is deactivated.
 
Yup - steam overlay fixed it. Runs pretty fantastic on my mid 2012 MacBook pro (2.6g i7, 8g ram, nvidia, SSD hard drive, Yosemite). But I'm still on my first 2x2km area. It does make the laptop run HOT so make sure you think about some type of airflow! :) Great game so far but I'm still just messing around and figuring out the dynamics. I can see already I'm going to fall in love with this game!
 
Turning off steam overlay fixed it for me too. Question though - anyone else with a 2012 rMBP (GT650m)? The game basically maxes out the CPU forcing the fans to max out and the laptop still runs too hot (over 100 degrees)...