Thanks guys for pointing out the "big" features of DX11 / D3D11.
Re Dx11 killer features: UAVs, resource views, compute.
Compute shaders is the big one. Combines with the ability for arbitrary read access from buffers and textures, it opens to door to all kinds of new techniques. We haven't even really scratched the surface of what's possible with compute. Other notable mentions:
Access to MSAA data
Access to depth buffers (possible in D3D9 through driver hacks)
Better instancing support (you can access arbitrary buffers now in vertex shaders, which makes instancing much more powerful)
Constant buffers (reduces CPU overhead)
Up to 128 textures
Decoupled textures and sampler states
Integer math in shaders
It's a shame that none of those things is possible to use solely within a HLSL shader - one needs an engine dedicated to DirectX 11 to make use of these features. This is what many shader dev's like me keeps "stuck" on DX9, as gasto says correctly:
That dilemma is what keeps developers(like me) from actually creating the engine. When I would have finished writing the engine in DirectX11, DirectX12 will be the cutting edge.