Bit Tech reports Microsoft decided that apps available in the Windows 8 store will need to be "appropriate for a global audience" and must not contain adult content. Perhaps some game publishers like Valve will be quite happy with this, as it will prevent a large subsection of the gaming landscape from being inside Microsoft's new marketplace. The site says games rated over PEGI 16 in Europe or ERSB Mature in the US will not be allowed by Microsoft, and will thus not be available as a "Modern app".
Although there is a possible workaround to this situation for some forms of media with the Windows 8 certification guidelines going on to qualify that apps that provide a 'gateway to retail content, user generated content, or web based content' are classified as Storefront apps which might avoid this requirement, the company has also confirmed that the rules will apply to all games submitted to the Marketplace.
These digital storefront rules do not stop mature games being installed on a Windows 8 PC completely and anything can still be installed as a desktop app in the same way that we are all used to. The rules do however prevent them from being sold through the Windows 8 Marketplace, which is the only way to buy software that works with the operating system's new user interface.