One thing that has largely unchanged with Windows for quite some time has been the installer. While just a small point, the flexibility of the installer to do things like unattended installations and service pack slip streams is very important. With Vista, Microsoft is radically updating the process. By transferring a pre-installed Windows image to the disk, Vista is able to boot itself into a fully-functional OS before you decide what it is you want to do. One particularly cool thing this will allow system builders to do is greatly improve how easy it is to build OEM images:

That way you can restore the image on multiple platforms. The caveat is that I wouldn't go from a 32-bit architecture to a 64-bit architecture, but staying inside 32-bit, you are no longer tied to the Hardware Abstraction Layer (HAL) any more, and that is a great feature that releases us from so many challenges we've had in the past with HALs and multiple images.
While none of this is useful for the end-user, for anyone working in a shop it's great to know. If you're a technician the article is well worth the read and goes deeper into the improved installer.