What Microsoft has done is shockingly stupid. Windows 7 is a very nice OS, but it's unusable in a touch environment. Yet Microsoft is promoting touch as the main user interface method on all Windows devices, from tablets to traditional PCs and laptops. It simply doesn't work, so to use the Windows 7 part of Windows 8 -- and Windows 7 is required for most of your traditional applications -- means you must have a keyboard and mouse. That makes Windows 8 on tablets an iffy proposition. If you have to add a keyboard and mouse to a tablet, you essentially have an assemble-it-yourself laptop with multiple pieces to misplace.
The stupidity doesn't stop there. The Metro UI is very touch-oriented, but it works poorly with a keyboard or mouse. People will eventually grow accustomed to using these input devices with Metro, but doing so requires relearning how to work with these devices -- and remembering to switch back to the old methods in Windows 7. Even if you have a touchscreen-equipped PC, where you might use touch for Metro and a keyboard and mouse for Windows 7, you're asking for users for way too much fundamental context-shifting.
Then there are the idiotic decisions in basic UI areas. For example, the Start menu is gone from Windows 7. That button is the central starting point for all of Windows, and now it's gone. Yes, you can use keyboard shortcuts to access its functions, but why? It's a bizarre throwback to a DOS mentality and not very helpful for touch users.
In Metro, the three menu bar areas (charms at the right, contextual options at the bottom, and settings and other functions at the top) are simply confusing. Why not one menu area to focus on? After all, that would greatly simplify touch selection. You'll also encounter unnecessary frustrations, like getting back an app you accidentally unpinned from the Start screen. I'm sure it's possible, but I haven't yet figured out how. Given how easy it is to unpin an app, it should be just as easy to repin it.
Then there's the confusion over Windows 8 on x86/x64 processors (WOX) and Windows 8 on ARM (WOA). WOA has only Metro, whereas WOX has both Windows 7 and Metro. That's related to the chip architectures, but users will be confused nonetheless. WOA doesn't support basic Windows management such as domain joining, forcing it to be handled differently by IT -- as if it were an iPad or Android, managed through a mobile device management (MDM) tool rather than as any other Windows client. That's just dumb -- and Microsoft should be able to figure out how to port the client portion of its admin capabilities to WOA.
These issues are different in nature than those in the mess that was Vista, where the two biggest complaints were a user interface that exploded with disorder and confusion, and a security model sure to frustrate users to the point of making it insecure. The basic problem with Windows 8 is that it has two essentially independent UIs: a traditional one for PCs and apps, and a new one for smartphonelike devices and apps. Yet you have to switch between them regardless of the type of device you are using.
Compare Windows 8's Jekyll-and-Hyde approach to the smooth transition Mac OS X and iOS users experience as they move from one device to the next. There's a commonality between the two, but desktop UI is not imposed on mobile devices, and mobile UI is not imposed on desktop devices. Apple has gradually trained Mac OS X users how to use touch gestures by supplementing Mac OS X Lion and the forthcoming OS X Mountain Lion with gestures, but it doesn't require their use and thus doesn't force the kind of awkward user experience that Windows 8 does.