I'm pretty sure a LOT of people are going to have the question "Will Diablo 3 run better on my Mac if I install it on Win7 in Bootcamp?". I did.
The question is fine, it was just poorly written.
First off, we need to clear up a misconception: Boot Camp is not a Virtual Machine. It is a boot loader and set of drivers that allows Mac owners to dual boot a machine between OS X and Windows natively. Because these two operating systems have very different underlying graphics libraries (DirectX is Windows only. Macs use OpenGL, and in some cases, a variety of Apples Core____ technologies), performance of a game ported to both systems can be dramatically different.
Complicating this further is the fact that some games are little more than the Windows version released with a WINE wrapper. Performance of these ports is often substantially worse on identical hardware. Others, like World of Warcraft, are written from the ground up to run natively under OS X, and might even have distinct features that take advantage of the platform, such as WoW's built in video recording and iTunes integration.
These questions are perfectly valid - the askers particular hardware configuration simply should have been edited out.
Normally we close "how well will this game run" questions for being too localized because almost nobody has exactly the same hardware configuration, and comparing mine to yours is an exercise in silliness because of all the different "moving parts" involved -- memory, video card, CPU, etc.
This case is different: It's the same game, running on the same hardware, with the only variable being the operating system. It's a valid question to ask, and like the post at the top says, one that applies to every Mac owner who plays Diablo III.
I'm not in his mind, but Raven has most likely simply enforced consensus expressed in this question in which we decide we don't want to handle with questions asking "how well" will a specific hardware run a specific game.
You probably have a point that "too localized" is an ill-fitting reason to close the question. What follows from this argument, though, is that it should be reopened and then closed again as "off-topic."
Now, it is true that an answer to this question might be valid for all hardware configurations of recent Apple computers, making the "too localized" point not applicable. I'm not too sure about that though and I'll pass judgement.
In our defense, the asker failed to write his question clearly enough and that contributed to the closure; thanks for suggesting an actually readable title.