Direct X is a platform introduced back in 1995 to persuade DOS programmers to write games that run on Windows. Over the 11 years that have followed, Direct X has evolved along with the massive improvements in hardware. Direct X 10 is scheduled for release alongside Windows Vista. A number of changes to DirectInput have been made, with the release of Direct X 10, Xbox controllers and peripherals will be supported in the code, allowing game developers to support the devices easily within the games.
Apart from this and minor changes in DirectSound, the biggest changes involve Direct3D. Expanded memory and texture limits (up to 8192x8192 textures, from 2048x2048 in DX9) are just the beginning. The biggest improvements are a unified shader model, which will hopefully free developers from having to write separate code paths for ATI and NVIDIA hardware, and new features such as the Geometry Shader that can add complexity to a scene procedurally. The Geometry Shader is integrated into the drawing pipeline, and allows the programmer to take simple shapes (such as triangles or even points) and generate complex shapes around them.
Microsoft has been a big fan of procedurally-generated graphics, including features on the Xbox 360 to specifically handle these items. However, DirectX 10 goes far beyond these ideas and adds features that, can't easily be done on the next-generation of consoles:
News source: Ars Technica