It's not a too big of a deal but you won't be able to play 18+ titles Adult Games on Windows 8’s Marketplace. With an age rating in place I personally don't see why Microsoft keeps restricting stuff as age restrictions on the operating system's marketplace, this would affect adult video games. A Microsoft representative confirmed with Kotaku today that, yes, section 6.2 of the Windows App guidelines applies to video games as well.
That section reads:
...apps with a rating over PEGI 16, ESRB MATURE, or a corresponding rating under other ratings systems... are not allowed.
For the United States, that's not exactly an issue. Not many major video games ever receive a rating beyond "Mature", the only notable exceptions being the PC version of Manhunt 2, the Director's Cut of Indigo Prophecy and, during the "Hot Coffee" scandal, Grand Theft Auto San Andreas.
But for other markets, it's a bit of a disaster. Europe especially. Here are some games that have, only recently, been awarded Europe's PEGI 18 rating, which under Microsoft's rules would mean they'd be excluded from the Windows 8 marketplace.
The Elder Scrolls V: Skyrim The Witcher II Call of Duty: Modern Warfare 3 Max Payne 3 The Walking Dead Sleeping Dogs Dishonored Mass Effect 2 Mass Effect 3 LA Noire Spec Ops: The Line Fallout: New Vegas Deus Ex Assassin's Creed: Revelations
So, yeah, nearly all of the biggest and best games released in the past three years. Good thinking there, Microsoft.
Of course, this doesn't ban the games from the operating system entirely. You can still install them and boot them up like you do now. It just means that, unless the publishers could be bothered cutting all adult content from their games, they won't be available from Microsoft's own store, and because of that won't be able to access a lot of Windows 8's fancier functionality.
I'm trying to think of a reason for this beyond Microsoft's US staff simply not thinking through the ramifications this would have on European publishers and gamers, really trying, but I just can't find one.
If you haven't yet read Muratori's piece, you really should, as it goes on to explain why this "bifurcating" of the operating system—in which a schism opens up between games supporting Win 8's marketplace and those which don't—could wind up a complete disaster for Windows 8 and Microsoft itself.
Of course, he's not alone in his overall thoughts on the matter: Valve's Gabe Newell suspects the OS could be a "catastrophe".