Here's another rumored feature of Windows Blue: The ability to bypass the Modern UI altogether and boot directly to the desktop. This shortcut would please long-time PC users who have little interest in learning new ways of doing things. But it could also hamper Microsoft's grand plan to provide a single UI for mobile and desktop devices.
Another concern: Would developers be less motivated to write apps for Microsoft's Windows Store if PC users are able to boot to the desktop, which won't run Modern UI-style programs? And if users avoid Windows 8's new interface, why should they upgrade at all?