Mac gamers have faster computers, fatter Internet connections... and more microphones... than Windows users
The first thing that jumps out is the average amount of RAM: a lot of Windows Steam users are still stuck with 32-bit Windows XP, and thus limited at 2 or 3GB of RAM. Mac OS X users, with their up-to-date 64-bit OS, enjoy at least 4GB. There are also more Mac users with multi-core systems -- again, probably because Mac users are early-adopters, while Windows gamers could still be using old hardware.
The only real discrepancy though, and one that won't surprise many gamers, is the crappy 9400M integrated graphics that still ships on a vast majority of Mac machines. Sure, integrated GPUs work, but you can be sure that those 32% of Mac gamers using the 9400M aren't running games at a high resolution! Once upon a time I was told to move from PC to Mac 'because of its superior graphics capabilities' -- perhaps that's the case on workstation Macs, but it wouldn't cost much to put a decent gaming graphics card in low-level Macs too! (I bet we'll be seeing some 'Gamer Macs' soon...)
Finally, the peripherals: microphones and Internet connections. It seems that Mac users, believe it or not, have faster Internet connections. This could be an early-adoption thing, but I think it's more likely that a Mac user is simply more proud of his hardware; his megabucks investment. When you spend a lot on a computer, you want to make the most out of it -- a decent Internet connection is cheap compared to a brand new computer, after all. To put it another way: I know a lot of Windows users that live in dank hovels... but Mac users are definitely a bit more well-to-do.
[Or maybe Mac users live in more affluent areas with better cable and DSL roll-outs...? Food for thought. Via Engadget]