Windows 8 Store won't allow PEGI 18-rated games
That's just the shop, not the entire OS.
The Windows 8 Store will not allow games rated PEGI 18. Dishonored, Skyrim, Mass Effect 3 - none would have been allowed.
"Your app must not contain adult content, and metadata must be appropriate for everyone. Apps with a rating over PEGI 16, ESRB Mature, or that contain content that would warrant such a rating, are not allowed," the Windows Store app rules stipulate.
"Windows Store apps are appropriate for a global audience."
This does not mean that 18-rated games will not be playable on Windows 8. Presumably games bought from Steam or Origin or elsewhere will work just fine. This is Microsoft's shop.
But the connotations for the future are scary. What if a Windows Store presence becomes a necessity for Windows 8 games? How many publishers could resist that user base (and how many users could resist Windows)? And then what - a world of PEGI 16 games?
There's a much more clever exploration of all this by software engineer Casey Muratori, whose work I found after reading Rock, Paper, Shotgun's take on the topic.