I should start by saying that I play on two Win 7 machines with the game maxed. The caveat being that these machines are 2 and 3 years old. My mobile option is a brand-new MacBook Pro that is having framerate issues. The Mac forums on battle.net are absolutely flooded with poor FPS threads. For an example, the Mac that I am talking about has a core i7 @ 2.9GHz, an AMD 6490, and 8GB of RAM. Such a machine in Win 7 would easily max out the game. Heck, you could probably run two instances of the game no problem. Instead, I turn shadows and AA off and still only get around 25fps gameplay. If I turn shadows to low, the framerate falls to about 10!
Imagine the frustration if you have a similar machine, let alone a machine from just a few years ago.
I should also mention that this MacBook Pro ultra's WoW vsynced at 60fps. Some will say that isn't impressive, but the number of particle effects and polygons pushed with an ultra draw distance is pretty insane. In bootcamp it runs Skyrim on high. It also runs PoE beta at 60 vsync.
The glimmer of hope:
On the first page, a blue owns up to the abject failure to code for Macs and offers the following nugget:
OpenGL 3.2, GLSL are features that we will use eventually and we hope to get better performance with them. We are currently looking for short term solutions that we can deliver to players as soon as possible.