Windows 8 Marketplace won't sell 18-rated games.
Microsoft has confirmed to Kotaku that section 6.2 of the Windows app guidelines applies to video games too. It reads: "...apps with a rating over or PEGI 16, ESRB MATURE, or a corresponding rating under other ratings systems ...are not allowed."
The rule won't impact US game buyers (unless they're hoping to pick up Manhunt 2 on PC or the Director's Cut of Indigo Prophecy), but it will be an issue in other markets.
Recent games to receive PEGI 18 ratings in Europe include The Elder Scrolls V: Skyrim, The Witcher 2, Modern Warfare 3, Max Payne 3, Sleeping Dogs, Dishonored and Mass Effect 3.
You'll still be able to play 18-rated games while running the operating system, but they won't be available from Microsoft's online store, which seems like a daft business decision.