People are still unsure exactly what to make of Windows 8 and its ability to run on any type of device. No one is really sure what to make of the fact that tablet optimized apps will need to be written from scratch, but legacy Windows apps will still run, too. This be-all approach may work for Microsoft, but Windows 8 will fail in the mobile space if it exposes the user to two areas of legacy Windows operation not appropriate for mobile devices.
I like what I have seen of Windows 8 so far, even though the exposure has been brief. The touch-friendly interface looks like a good fit for tablets, and it looks refreshing. Of course, 'refreshing' was a common description of the Windows Phone 7 interface, and that hasn't set any sales records since its release. Hopefully Windows 8 will have a better reception for tablets.
It is not clear how much of the legacy Windows core will be underneath the Windows 8 pretty stuff, but if the "works on everything" approach means that third party device drivers will be part of the equation I don't think the tablet consumer will tolerate the mess that comes with that. Mobile device users expect the systems to work out of the box and don't care how that happens. They don't care that vendors of hardware components have to write device drivers to make Windows operate properly with all of the parts of the system. They don't care that the device driver for each component has to interoperate with the device drivers for every other piece of the system.
No, tablet owners are going to expect the system to work as expected out of the box, and to keep working forever. No future system update can force a device driver to affect proper operation and thus require some nameless company to produce a driver update. Everything must work, and without fail or Windows 8 will be a big flop on tablets.
The Windows legacy may expose Windows 8 to the need for anti-malware protection, and if so this will not work on tablets. Microsoft must realize that mobile device owners will not tolerate regular software updates, even to update the anti-malware protection. Anything that exposes the owner to interrupted use of a tablet is going to be interpreted as a failure by Windows 8.
It is not clear if Microsoft is rewriting Windows 8 from scratch, but with so much legacy code invested it's unlikely. Doing so for the mobile version of Windows would have been a good idea in my opinion to ensure the tablet user experience in no way resembles that of Windows past. Tablet owners expect to spend no time on maintenance, updates or dealing with unresponsive systems, even if it's the fault of third parties. Tablets must work out of the box, and work as expected every time the user takes it out. Otherwise Windows 8 will not fare well in the tablet space.