We only just reported that DirectX 12 will be providing CPU-bound games with up to 50% more performance, but the new version of Microsoft's API will be completely exclusive to Windows 10.
This is going to hurt some gamers, but with the offer of providing Windows 10 for free for Windows 7, Windows 8 and Windows 8.1 users, the sting won't be felt for too long. We shouldn't be too surprised, as previous versions of Windows have had exclusive rights to the latest version of DirectX. Windows Vista had DirectX 10 for example, but let's not even begin to talk about Vista.
The reason behind DirectX 12 being exclusive to Windows 10 is that Microsoft is making the desktop OS free to everyone who owns the last three versions of its OS. So anyone who wants to get in on more performance, or better quality graphics (DX12 games) then there is nothing bad here at all.