The big winner here is of course Windows 8, where mature titles such as Grand Theft Auto 4 can be distributed for Windows 8 devices, but this change will also allow mobile developers to look to more adult titles in the Windows Store for Windows Phone.
PEGI is a European initiative to have consistent labelling across countries as to the suitable of the content of a game title for various age groups. It only comments on the content and depictions in a title, not the perceived skill level or suitably of a game to an age. PEGI 18 is defined as follows:
The adult classification is applied when the level of violence reaches a stage where it becomes a depiction of gross violence and/or includes elements of specific types of violence. Gross violence is the most difficult to define since it can be very subjective in many cases, but in general terms it can be classed as the depictions of violence that would make the viewer feel a sense of revulsion. Descriptors shown on the back of the packaging indicate the main reasons why a game has received a particular age rating. There are eight such descriptors: violence, bad language, fear, drugs, sexual, discrimination, gambling and online gameplay with other people.
It's unlikely that there will be a flood of violent free-for games such as Grand Theft Auto or Carmageddon on your Windows Phone, but the option is now there for developers to explore more adult concepts in their Windows Phone titles.
Titles like Car Jack Streets (Tag Games) could now be more realistic
Ted Dworkin (Microsoft's Director of Program Management, Windows Store) covers this in more detail on his blog post.