Like AMD's Mantle API, it promises reduced CPU overhead and lower-level access to graphics hardware. But DirectX 12 won't be limited to one vendor's hardware.
Mantle will work on this hardware as well, and won't be limited to one vendor's operating system. From AMD's Mantle FAQ
The time spent on any item of the agenda [of a finance committee] will be in inverse proportion to the sum involved. -- C.N. Parkinson