Microsoft revealed that in addition to making lower-lever abstraction available on existing hardware, DirectX 12 will also introduce new features for next-gen GPUs. The Tech Report writes that among other things, these new features will include new blend modes and conservative rasterization, which can apparently help with object culling and hit detection.
DirectX 12 will indeed make lower-level abstraction available (but not mandatory—there will be backward-compatibility with DX11) on existing hardware. However, Tamasi explained that DirectX 12 will introduce a set of new features in addition to the lower-level abstraction, and those features will require new hardware. In his words, Microsoft "only teased" at some of those additions this week, and a "whole bunch more" are coming.