Will be very different.
Had a meeting last week with MS, but unfortunately had to sign an NDA. But I can share what's already been leaked publicly.
1) It will run on both ARM & x86
2) There are two UIs to choose from, which are radically different: "Immersive" (tablet/touch UI which is similar to the 'Metro' UI on Windows Phone), and a Windows 7-based UI
3) There will be an "app store", but it is not mandatory.
4) Windows 8 binaries can be in a new package. Not an ".exe", but an ".appx" file. These are single files which are containers for all necessary program files, and they are .NET. This means a single "appx" app will run on both ARM & x86 Windows installs, and tablet/desktop installs
5) The Explorer UI is Ribbonized. Not sure if I like it.
More to come.