What @EdamllamaB said: multithreading
DX12, graphically, has nothing more than DX11 has. In fact you can alternately select DX11, DX10, and DX9 feature sets - but there is no “DX12 feature set”.
But multithreading in DX11 is more primitive and limited. DX12 essentially removes the top layer of the API, exposing a more direct hardware interface that is MUCH more complicated and detail-ridden, and quite frankly really unpleasant to program. But much more robust at handling multile threads.