If no one has ever noticed, Windows 7 was really the first Windows OS to be touch centric. It brought in more touch gestures than ever before, the Taskbar size was increased so touch users could easily launch items in the Taskbar and switch between items easily, even the window controls are larger than they were in vista for touch screens. But tablets running Windows 7 never took off, probably because of the Desktop. It's not very mobile centric. Windows was never really built like a mobile OS. The Desktop isn't a mobile OS.
I think that might change with Windows 8.
It has elements of a mobile OS: it's more touch centric, it's commanding than others mobile OSs, it has an easier to use interface. Having said that, one of the things no other tablet has is a Desktop. They also don't have the ability to integrate well with established networks. You can't print a word document or create a fantastic PowerPoint slide show on an ipad or android. On a Windows 8 slate, you can. A major selling point of a Windows 8 slate will be the fact that you can use apps that are designed like the OS, and are touch centric. Then, you can go into the Desktop to play a PC game or edit a photo with PhotoShop, or even use iTunes to manage the media on your ipad.
With Windows 8, it builds off the current touch features of Windows 7, and expands that into a proper mobile OS for tablets and slates while also being an operating system that DOES work with a keyboard and mouse.