For a taste of the advantages of a 3d OS interface, check out The Croquet Project. It seems to be an attempt at fulfilling all the promises of Java along with all the promises of large-scale networking: an OS environment that can be shared by anyone, completely modifiable at run-time, bit-perfect interoperability across platforms.
The major flaw I see is its reliance on OpenGL - even though in my experience I've had better luck and smoother operation from OpenGL-based applications. I've heard that current versions of DirectX like to overwrite any OpenGL files with special wrappers that lower OpenGL's efficacy. Not sure how true that is, but it sounds likely.
Then there are the usual concerns of what kind of system specs a person would need in order to run anything complex. And convincing people to go to an unproven system incompatible with lots of other programs. Also, it seems to require adjustments to programming practices, and that can be a difficult task.
Still, it seems much more extensive, user-friendly and workable than Java3d or VRML.
I also checked out the Windows Vista page, just to see what could possibly drive me to upgrade almost everything in order to run something that's going to, no doubt, force a whole bunch of DRM bullshit on me and leave the usual gaping security holes.
I'll say this - interesting but not compelling. The entire style is so clearly copped from Macintosh that I wonder who got paid to come up with the "design." Judging from the graphics and search options, I imagine this will be a hard drive and RAM hog, more so than the current version.
And five different versions? Not only confusing for consumers, but stupid in terms of compatibility for upgrades and customer service.
The features they offer aren't bad, and they'll be nice for people looking for them all in one place without having to look around the Internet.
The new Windows explorer will have tabbed browsing? So does Firefox (which is, y'know, free). The Sidebar will feature cool gadgets that you can install? So does Firefox. And my Google/Yahoo home page. What about tagging files? I've found the practice to be unnecessary on my PC - but if I wanted to, XP already has that functionality. Right-click a file, go to properties, select Summary and, if necessary, click Simple. Now add your metadata and use something like Copernic Desktop Search to index your files.
Basically it's going to be a flashier, even more bloated version of Windows with functionality that you can find in free, less-processor-intensive programs.
Not a big surprise.