The changes are a complete re-write. One of the major changes is that it is based on OpenGL and not DirectX. Another major change is .NET 4.0 instead of .NET 2.0 which allows a lot of dynamic compilation features that are heavily used. The other major change which is where a lot of work is currently being focused on is the inbuilt configuration utility.
The software is headed towards FuseGL. MDX will be completely dropped and unsupported in the near term. Long term, you never know. But Microsoft has a good track record of backwards compatibility and as long as they continue to support .NET (which I see no immediate chance of it being dropped) and video chip vendors continue to support OpenGL (again, another big one) everything should be ok. Windows 8 supports .NET and OpenGL so we are good to go there. Since Windows 8 is still in development nobody can no for certain anything will be compatible. The most we can do is guess based on past events, and the last time an OS change caused things to not work was the 98 to ME transition where they dropped support of the old driver style. So again, I think it is safe to say I am ready for the future.