50% less CPU use for the main thread.

Mantle has the same CPU savings, and during discussions on that game developers have talked about how in many (most?) games, up to 50% of your CPU time is spent in the drivers. Considering how many games I've run into lately where single-core performance is the bottleneck, this doesn't surprise me at all.

Mantle and DX12 strip out a lot of layers, optimize a lot of what it keeps, and spread a good chunk of the remaining load over multiple threads, meaning that instead of trying to use 120% of core 0 and 15% of cores 1-3, you can use 60% of core 0 and 25% of cores 1-3 (just as a hypothetical made-up example).

Greater efficiency, more ways for devs to optimize their games. Should see some FPS improvement with dx12.

Better multi core utilization. Less cpu use. Less dependency on single threaded performance.