I was particularly hoping for improvements on DirectCompute but nothing was mentioned. For example, DX11 lacks dynamic parallelism as seen in CUDA and OpenCL 2.0. So far it looks like DX12 was focused solely on graphics.
As for current hardware support, it may very well be a "feature level" type of scheme similar to DIrectX 11. In DirectX 11, runtime is separated from the "feature level" so you can have DX9 GPUs supporting DX11 runtime and APIs, just with a declared feature level of say 9_3 so some features may be disabled but at least programmers can target a wide variety of hardware with less effort. They have announced that DX11 hardware will support DX12 but that does not mean everything will be necessarily supported in current hardware. However, it is not clear if there will be a new feature level called say 12_0 for DX12 and what, if anything, it will bring.