This should make things easier for software developers, while at the same time allowing hardware designers to approach things in the manner they see best.
Although DirectX 10 compliance does not require unified hardware shaders, the driver interface will be unified. Microsoft will also be implementing some technology from the Xbox 360, enabling the practical use of unified shaders like we've seen on ATI's Xenos GPU for the 360. DirectX 10 will be bringing support for a new type of shader, the geometry shader, which allows for the modification of triangles in the middle of rendering at certain stages. While we'll have more on the new hardware features supported by DirectX 10 later this year, we can talk a bit about what we know now. The benefit comes in the advanced features made possible. Upgrading to Vista and DX10 won't absolutely require a hardware upgrade. We've only recently begun to see games come out that refuse to run on DX8 level hardware, and it isn't likely we will see DX10-only games for several more years. New games which use DirectX 10 under Vista while running on older DX9 hardware will be able to gracefully fall back to the proper level of support.
It has been a while, but this transition (under Vista) will be no different to the end user than the transition to DirectX 8 and 9, where users with older DirectX 7 hardware could still install and play most DX 8/9 games, only without the pixel or vertex shaders. There seems to be some misunderstanding in the community that DX9 hardware will not run with DirectX 10 installed or with games designed using DirectX 10. It is likely that driver support will allow for DX9 compatibility, while new feature support could easily be added through OpenGL caps, but the exact steps ATI and NVIDIA will take to keep everyone happy will have to unfold over time.
This may pose a problem for users that want to upgrade their hardware without upgrading their OS. DirectX 10 will not only include support for new hardware features, but relies on some significant changes Microsoft is making to how Windows treats GPUs and interfaces with them, requiring the clean break. Of greatest importance, DirectX 10 will be a Vista-only feature Microsoft will not be backporting it to XP. Unlike the previous iterations of DirectX, 10 will be launched in a different manner due to all the changes to the operating system needed to support it. Previously, Microsoft was doing pretty good at providing yearly updates to DirectX.
DirectX 10 has enjoyed an odd place recently in what amounts to computer mythology, as it has been in development for several years now while Microsoft has extended DirectX 9 to accommodate new technologies.
Visual changes aside, there are numerous changes under the hood of Vista, and for much of our audience, DirectX 10 will be the biggest of such changes.