One thing I always point out in the Vista flyers is that they advertise how Vista will be "so much more secure", but they never say how. If MS would release DX10 for XP, then Vista would be totally pointless.
Unfortunately, only John Carmack is smart enough to use OpenGL as his primary platform when developing engines. Everybody else uses DX, with the exception of the Linux version of the Unreal engines. IMO, OpenGL should be the standard because it is a lot more efficient and is tons easier on the graphics card than DirectX.
People will purchase Vista the same way they purchased XP, when buying a new computer. People upgrading to a new computer usually don't take OS into consideration, they don't know one version from the other.
I think Windows XP was the best Windows release in history. DX10 aside, Microsoft is going to have to include more in a new version than just a fancy skin to make it a worthy upgrade.