Real touch screen support and an app store.
I know that it may seem like not much given that it's focused in the Metro UI, but I think it can be a lot bigger than many here suspect.
I've already had a friend ask me about the Sony Vaio Tap 20. A 20" all-in-one that has a battery so you can take it to the kitchen or coffee table. All for $900.
Windows 8 gives you the ability to have the desktop experience, which is at least as good as Win7. But also to have a first class touch experience on the same device.
It fixes the problem I have of taking a tablet and a laptop on travel. I need my laptop for work, but the tablet for everything else.
Right now it may seem like a contrived example. I think in a year Apple will be doing the same thing. And in 18 months we'll have trouble imagining why this was at all controversial.