One of the new features of Windows 8 is DirectX version 11.1. DirectX is the Windows component that is used by video games to address the graphics card. One of the key properties of 11.1 is its support for 3D monitors. This means that no additional software is required to play a game in 3D on a compatible monitor.
Microsoft employee Daniel Moth reports on the Microsoft Answers Forum that will remain exclusive to Windows 8, and that the latest version will not be released on Windows 7. Although it's unlikely that there will be DirectX 11.1 only games in the near future, it does mean that an upgrade to Windows 8 is required to play them.