DirectCompute and multithreading support, too
DirectComputeMulti-teraFLOP GPUs pave the way for running complex calculations at speeds greater than any CPU can execute them. That's a reality and not mere hyperbole. The problem is in how developers harness the power on tap, especially for non-gaming tasks.
DX11's compute shader enables the use of the GPU as a general-purpose processor for (mainly) non-graphical tasks such as stream computing. What this means, gaming-wise, is part of the GPU can be apportioned to handle physics, shadows, or AI. Looking farther afield, video transcoding is particularly suited to being executed on GPUs.
The intrinsic beauty of Microsoft pushing DirectCompute into DX11 is that it will work on both ATI and NVIDIA cards (should they support DX11) and any application run via the compute shader will be company-agnostic.
Multithreading
Most DirectX games are single-threaded. What that means is that only one CPU core is charged with providing the GPU the necessary instructions for execution. That's fine for older CPUs but most modern chips provide multi-thread support, right up to eight on the Core i5/i7 processors. DX11's multi-threaded rendering (display lists) leverages additional CPU resources for better overall performance.
DX10 supported, too?
Owners of DX10-class hardware needn't panic that their graphics cards will miss out on many of DX11's features. The API has a fallback to DX10, where the features are supported with limited functionality. The one exemption is tessellation - a DX11-only feature.
Lastly, HDR compression now uses what are termed BC6 and BC7 texture formats. BC6 introduces a 6:1 compression of high dynamic range textures, although it's not lossless. BC7 works losslessly on low dynamic range formats.
Games
DX11 is technically better than DX10.1 in a number of respects, there's little arguing that fact. Most readers won't care about the DX11 'checkbox' next to the Radeon HD 5850/70 GPUs if games aren't readily available to take advantage of the API. At the time of writing, DiRT 2; S.T.A.L.K.E.R: Call of Pripyat; and Battleforge are confirmed for release in 2009.
