To the OP, that's because a Windows OS on Mac hardware runs better than a mac OS on mac hardware...I have actually run benchmarks and it's hilarious that windows takes better advantage of the mac hardware than OSX does :)
My 2008 macbook with 4 gigs of memory can run this on ultra/high (lowered shadows and shaders) just fine. The only problem I noticed is that there seems to be a memory leak. I have to restart the game after 3-4 hours or it gets unplayable.
On my windows partition I can run it on full ultra without the memory leak. Blizzard just needs to optimize better.