Here's my thoughts, in quick bullet point format:
It will be huge. It's Windows, FFS.
Windows 7 is brilliant, on the desktop, and MS was so badly burnt with Vista, they will not let it go backwards, so I'd expect Windows 8 to be ask good on the desktop as Windows 7, with tablet stuff added on.
The tablets will do as well as Android tablets. Which is to say: fanboi's, and a few others. Thats it.
Laptops, and maybe convertables, will rock with it, even with touch screens on them.
Which leads to the inevitable comparison with Apple.
Microsoft is taking Windows (the whole thing) and shoe-horning a new UI over the top, but keeping the old UI. They did this (well, HTC did, technically, with Sense) with the "shells" on Windows CE / Windows Mobile 6.5. It looked great until you hit an app which wasn't Metro/Sense, at which point it was like hitting a brick wall. Mail was one such application. I expect Win8 will be the same. Office, I suspect, will be a prime example of this brick wall.
Apple did it the other way. It's a new OS and form factor, based on the same underlying OS, and you need new apps. These new apps need to be redesigned for the new form factor, tho if you use "our tools" (XCode, or VS + .NET + WPF in the case of MS), it'll be pretty easy for you to move over.
Personally, I prefer the Apple way, which should come as no surprise. An iPad, for example, is not a Mac, so it would be broken to run Mac desktop apps on it. Even if the underlying OS (Darwin/OSX), frameworks (Cocoa/CocoaTouch) and other stuff is the same or very similar.
Microsoft could do the same. But it will not. You could have Windows 8 for the desktop (same as Windows 7). You could have Windows 8 for tablets, which will only run the new Metro UI-based apps. If you want apps for the new tablet, write them in .NET/WPF/Metro, and get them from the AppStore. But the tablet only runs those apps. You can't run "any" desktop apps on a Tablet device - at least not by default.
It'll be interesting to see what happens. It IS, however, time for me to learn WPF properly. Now, if I could do WPF (or something very similar) on iOS, with MonoTouch.....
Some reading (thanks to Dan):