Downloaded and installed the Windows 8 preview to give it a try.
My initial impression with setting up the OS was quite nice – the UI seems nicely put together. Once the machine is configured, I was dumped at the new ‘Metro Start’ screen.

The look of the new start screen is interesting at first. But a few moments actually trying to use the interface with a mouse reveals a few major issues.
It’s painfully evident that the Start screen is designed for a touch enabled device, with hardware back/home buttons. That is: Tablets, or mobile phones.
Anyone using a keyboard/mouse is going to get frustrated and/or lost very quickly. The only way to navigate is to find the scrollbar hidden at the bottom of the window and drag it (which disappears if you keep your mouse still).
This touch optimisation (or rather: exclusivity) extends to the new Metro style immersive applications. When you launch the new application it completely takes over your desktop, with a full-screen interface.

Dude, where's my multitasking?
Once you’re inside the application, there’s no UI elements indicating how to get out. Yet another indicator people are going to need to use Windows on a tablet from now on.

Dude, where's my exit/back button?
It is possible to bail out using just the mouse – so far I’ve found that putting your mouse on the far left edge lets you click away to another application. And eventually, perhaps, to the Metro start screen. You have to actually press the Start key to get back home reliably. (Seriously, wtf?)
Back on the home front, clicking on the Desktop takes you to what you’ve come to expect from Desktop OSs of the last oh 20 years or so.

Gah, what the hell... they've added the Ribbon UI to Explorer? 20% of the window real estate gone to this.
This brings up another point of frustration with Windows 8. They’ve now effectively got two completely different ways of dealing with applications.
The “Classic” windows way lets you have windows (whoa, crazy) and to quickly switch between tasks using a mouse.
The new “Metro” way lets you have only one task open at a time, and switching requires a keystroke of some kind.(I’ve been told you can have two apps side by side in an 80-20 split, but I havn’t figured that out yet)
It also means that you’ve got two sets of applications – and to use them together is nigh on impossible.
Perhaps in multi-monitor scenarios you could have the two combined – but again, that the Metro UX is entirely touch focussed means those using a mouse are going to have trouble. This brings into contrast that those using Windows8 on a tablet are going to have trouble using “Classic” windowed applications which expect relatively fine control from a mouse.
I don’t see the purpose, other than as an elaborate practical joke, of combining the two.
Shut down? You don’t need no steenking shut down options.
Lastly, figuring out how to shut down or restart is an ordeal. In every version of Windows back to ’95 you click Start, and then Shutdown (or a power icon).
In Windows 8, either this has been forgotten as a task users do with some regularity – or it’s being deliberately obscured because on a Tablet you don’t need to.
In any case, it’s a multi-step process requiring you to click Start, then your Profile, select Log out. Then windows goes to a tablet/mobile style “lock screen” which you need to (no shit) drag up, so you can hit the power option ,and finally select shutdown.
Conclusion
My only conclusion from a few hours poking around is that Microsoft are deliberately abandoning the desktop, or have decided that since Windows Phone 7 was such a raging success (yes, that’s sarcasm Sheldon) they’d go copy/paste all over the new version of Windows.
Seriously, I have this vision in my head of the design meetings for Windows 8. They’re all in mutual admiration that they produced a Phone OS that doesn’t suck as much as Symbian*. Then someone says “Well hey, it worked on the phone, lets make the next version of Windows ALL ABOUT THE TILES”.
I can only hope that someone realises they need to target the Tablet and desktop seperately before they launch, or they’ll realise that despite awesome engineering in the backend – they really can repeat the apparent disaster of Windows Vista.
* Okay, so Windows Phone 7 is actually pretty nice. I’m just venting, ok?