A few weeks ago, Microsoft told everyone that they would not allow content that had a rating over PEGI 16 OR ESRB Mature into their Windows Store. Due to the difference between the US' ESRB and Europe's PEGI rating systems, this meant that a lot of games available in the US wouldn't be allowed in Europe. You could of course still run the games, just not buy them from the Windows Store.
It seems that Microsoft realized its mistake, however. The Windows corporate VP of web services Antoine Leblond told Gizmodo, “It basically ends up disqualifying games that would be ESRB Mature." Yeah, big surprise there Microsoft.
They've since revised the policy to allow Mature and PEGI 18+ games to be sold through the Windows Store. The change won't come into effect immediately, so it may take a while for the games to show up on the storefront.
Windows 8 and the changes Microsoft have been making have been increasingly exclusive, trying to convince (or force) users to purchase and do everything through Microsoft as their one-stop-shop. This approach however, has scared away some developers from releasing anything through their store already, so it remains to be seen how this will work out for Microsoft.
Source: Gamasutra