Metro is designed for tablets
As I detail in my review, "A deep dive into Windows 8 Developer Preview," Microsoft heaped all of its creativity in Windows 8 on the new Metro interface, which you boot into when you start the operating system. There's a lot nice there, but it's been designed for tablets, not PCs. Not everyone will like working in an operating system designed for touch when all they've got is a mouse and keyboard.
There's nothing much new on the Desktop
Windows 8 relegates the Desktop to being just another app in Metro. When you get to the Desktop, it looks and works just like Windows 7 --- and in some ways is worse. The Start button, when clicked, doesn't reveal a menu from which you can run apps, open documents, and so on. And if you want to use the Desktop Control Panel, you'll have to switch back to Metro, move your mouse pointer to the lower left portion of the screen, select Settings, scroll to the bottom of the screen, and then select More Settings. In Windows 7, the Control Panel is available right on the Start menu. For those who live in the Desktop, Windows 8 doesn't seem to offer any benefits over Windows 7, and may even be harder to use.
The interface is confusing
Windows 8 is essentially two operating systems, not one, bolted together in a not-very smooth way. Metro is designed for tablets; the traditional Desktop is for PCs and laptops. There's very little connection between the two; the interfaces look different from one another and work differently from one another. If you like a seamless, integrated operating system Windows 8 might not be for you.
Microsoft will control what Metro apps you can download
If you want to download a Metro app to run in Windows 8, you'll only be able to do it via the Windows Store. This breaks with the long-standing Windows tradition of allowing people to download any app they want. That means that you'll only be allowed to download Metro apps that Microsoft says you can run, just like Apple does with the App Store. In essence, this is a form of censorship. You'll be able to download any app you want to the Desktop, but you can already do that in Windows 7, so why bother to go to Windows 8?
It's trouble for businesses
Businesses will face serious problems upgrading from Windows 7 to Windows 8 because of Metro --- it could require retraining users, countless help calls to the Help Desk to aid with Metro problems, and deployment woes. Given that Metro is designed for consumers, not businesses, it's not clear what benefit businesses will get out of Windows 8. They'll likely stay away.