I feel the same way; what we've seen of Win8 marks the first rethinking of desktop computing since the original WIMP (Windows, Icons, Menus, Pointers). (Not the first, actually, but the first to actually see the light of day from a vendor as big as MS). And the fact that it's HTML5-based doesn't hurt. :)
What exactly are they rethinking? To me it looks like a desperate attempt to catch up with the tablet trend without leaving their massive existing customer base out in the cold.
So what do you suggest they do? Stay with a stagnating UI that's not suitable for anything other than a desktop in fear of looking "desperate"? The UI they pulled off is far from desperate, in my personal opinion it beats everything available right now in the mobile world as far as usability and looks go. For example, I haven't seen any one call the new windows phone UI anything short of amazing and it's the same concepts that will be applied to the Win8 UI from what I've seen so far. Their approach also seems much more likely to remain unified across multiple touch device form/scale factors when compared to something like Android.
As far as being last to arrive at the scene, need I remind that this is Microsoft? They're always last, and sometimes that actually works out for them (XBox, .NET), and sometimes it doesn't (Bing).
I grudgingly have to agree with you here. Neither iOS nor Android really radically break from the UI conventions of the desktop. Microsoft's latest UIs are both bolder and more elegant. And with C# they have the best development stack by far of the three.
They're definitely cribbing some from tablets, but they're also doing so using WinPhone7, which was itself an innovative take on touchscreen phones (at least, compared to the Android/iPhone norms). But I think the overall idea of subdivided rectangles with large touch targets is a pretty interesting one. (I'll be less excited if they stop there.) :P
