The driver performance just isn't there. If you have a cross-platform game and run it on the same Apple machine in OS X and on Windows, the Windows version's gonna be WAY more performant.
As a lesser frustration, most Windows games I play natively support the Xbox 360 controller, whereas almost no OS X ports do.
I was able to play quite a few games (Civ5, Walking dead, Portal 2, Diablo 3) without any issues on a Macbook (Pro 15 Retina first generation). Portal 2 and Walking Dead are quite playable also in my Macbook Air 13 (2010, last model with nvidia board).
Of course these are relatively low requirements games. Do you have examples where the performance difference is relevant, and the games actually unplayable ?
On my mid-2012 11" MB Air, most games around the level of requirements you said tend to run maybe 3/4ths as well in OS X as in Windows 8 (Portal 2 being a concrete example). Not unplayable by any means, but the performance difference is substantial enough that it's worth my while to reboot into Windows.
You're right, but it's moving fast (At least once a year...). I would say that native games are 95% on par with Windows equivalent.
Wine and other "Translator" are moving fast too, in a few release you'll see a lot of big improvements in the graphics area. Crossover latest release already profit from that.
It's been a few years since I tried, but I got much better performance running the Windows version of the Orange Box in Crossover than I did with Valve's OS X port. Is the driver situation significantly different when running in Crossover/Wine than native?
I bought mine refurbished for 1300 euros, which is comparable to the price of similarly specced PCs (and about the average salary of most European countries).
Even if it was three times this price, what has that to do with its ability to play games ?
Playing AAA titles might be difficult if they're badly optimised/don't have low graphics options. Others, like Skyrim, can run on an 2011 Macbook Air if you put the settings to low(though it leaves you with a surprisingly boring experience once the "HOLY CRAP MOUNTAINS!" is taken out of the equation).
If you're into indie games/GoG oldies it makes literally 0 difference what hardware you run most of them on as long as they actually run on that OS.
As a lesser frustration, most Windows games I play natively support the Xbox 360 controller, whereas almost no OS X ports do.