I'm reminded by the interviews with the original Xbox team at MS, who were apparently shouted at by a red faced Ballmer for weeks after they said that it wouldn't run Windows. Imagine how the Xbox's usability and popularity would have been reduced if they tried to run the Windows desktop on it.
It seems to me that there is a huge push at the upper levels in Microsoft to put Windows on everything.
Meanwhile, Apple realized the sad but true fact that most computer users are completely incapable of understanding concepts like the filesystem. They made a tablet that your granny can use without having to be taught. They made a system as foolproof as possible, while the power users who scream "walled garden" and "i needs my zsh shell on it!" can still use it as a casual device and jailbreak it if they want to get the features they require.
Many in the tech media lambasted the iPad as a useless oversized iPod touch, but look at it today. They shipped a "crippled" device that ended up being more usable to more people than the PC has been. They also successfully convinced users who want to be able to do things like spreadsheets and presentations to try things like Numbers and Keynote, which were completely redesigned for touch and work great on the device.
Microsoft hasn't realized this strategy, and I think many users will be worried by the initial lack of apps for Metro and try to use Word and Excel on a 10 inch touch device, and get angry with it when the UI is just too cumbersome.
I played around with a Windows 7 tablet at PC World recently. An anti-virus warning from McAfee or something like that was popping up and becoming modal every time I tried to open the Start menu, then when I eventually managed to kill it, the items in the Start menu were way too small to accurately touch. Text input worked like crap, and IE was barely usable. Anyone buying that device was doomed to constant frustration. (At the same time, a friend of mine, who is a hardcore Arch Linux user and a sysadmin, went in with the intention of getting an Eee Pad Transformer or other android tablet, and was so disheartened by the user experience of those that he left with an iPad 2).
Really, I don't want Microsoft to fail. Many people have hated them for many years, but these days it seems to be more pity than hate. If they released an excellent update to Windows for the desktop, still focusing on the mouse and keyboard, then pushed for developers to make awesome apps for Metro and had that only on tablets, I think they would be much more likely to gain some marketshare, especially if they pushed some cloud syncing features and integration with other products. Of course Windows 8 will gain marketshare anyway, I think mostly through OEM sales, but I don't think the tablet market will pick it up like it might if they focused on a solid tablet-only OS but played the vertical integration game with services.
Just my opinion.