If you are a PC gamer, DirectX has had an impact on your playing experience. Going head to head with OpenGL for many years in a row now, DirectX has been a huge leverage tool for Microsoft getting people, developers included, to stick with Windows. That's why this recent interview with Chris Donahue is so interesting. Going over primarily the role of DirectX 10, Donahue covers many of the new developments DX10 brings, along with its integration into Windows Vista. He talks about backwards compatibility with newer games and people who don't have DX10 capable cards. Since physics has become such a big issue, this answer regarding its role in such was interesting:
Windows Vista will support Physics solutions of all forms. Physics engines can execute on CPU, GPU or a custom hardware. With the support for HLSL and DirectX 10, Windows Vista has an awesome platform to enable Physics on the GPU. Multiple GPU support (LDA - Linked Display Adapters e.g. SLI and Crossfire) enable game engines to distribute their graphics and physics loads across multiple GPUs.
The interview is quite lengthy, and is heavily Microsoft oriented, but is well worth the read. A list of some "showcase" DX10 titles is at the end, including Halo 2 for the PC, which many people have been wondering about for a while.