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.
http://techreport.com/news/26210/directx-12-will-also-add-new-features-for-next-gen-gpus