A different look with some new features and a serious lack of some old ones like the "add/remove Wndows components" options for setup in the add/remove programs section of the control panel. That's just for starts there. It's solitary advantages seen over XP is the automatic repair of Windows when booting from the installation disk over a recovery console, memtest and Windows Defender are now included, and the ability to reinstall it's own boot loader.
The installer will no longer allow you to create a custom sized primary with the total drive space number lowered. And to add a second OS you first have to shrink the Vista primary since Vista wants the entire drive to start off with. Did I say enough? The wait for more driver support along with the eventual service packs or 2nd edition can also be added.