In an interesting move, and one sure to disappoint a number of European Windows 8 users, Microsoft will be prohibiting any apps that contain adult content from appearing on the Windows Store. Specifically, any games that have acquired a rating over ESRB Mature or PEGI 16 will not be allowed to appear on the storefront; neither will games that "would warrant such a rating".
For Store shoppers in the United States, this isn't such a problem as few games end up with a rating above Mature. However, for European gamers, a whole range of games will not be able to be sold from the Store due to the fact that they received a PEGI 18 rating (above the allowed PEGI 16). Some games that would be prohibited include The Elder Scrolls V: Skyrim, Call of Duty, Assassin's Creed, Borderlands 2, Dishonored and Max Payne 3.
This issue was highlighted by programmer and blogger Casey Muratori, who cites Section 5.1 and 6.2 of the Windows 8 app certification requirements:
Apps with a rating over PEGI 16, ESRB MATURE, or that contain content that would warrant such a rating, are not allowed. Metadata and other content you submit to accompany your app may contain only content that would merit a rating of PEGI 12, ESRB EVERYONE, or Windows Store 12+, or lower.
Now this doesn't mean you are prohibited from installing these games on your Windows 8 machine in Europe - in fact you can install whatever you like outside of the Windows Store - but it does help fragment the PC gaming marketplace and prevents a one-stop shop for all Windows games. It also prevents high profile, but mature content games from receiving the exposure they normally would from the Windows Store.
Muratori also highlights some other sections in the certification requirements that may present problems for people submitting content. According to Section 5.3 your app must not contain content that "encourages, facilitates, or glamorizes illegal activity"; Section 5.6 says that you can't glamorize "excessive or irresponsible use of alcohol or tobacco products, drugs or weapons"; and Section 5.8 says "your app must not contain excessive or gratuitous profanity".
It's interesting that there will be a major discrepancy in allowed content between European and United States markets, based on the fact that a majority of US games get a 17+ rating whereas their European counterparts are restricted to those 18+. Keep in mind that these restrictions only apply to the Windows Store, and gamers will still be able to install adult games through other storefronts such as Origin and Steam.