Dmitry Fadeyev writes about Windows 8:

The road to a good OS is not a blurring of the lines between PCs and tablets, but rather an amplification of the differences through a strong focus on the uses that each category serves. The desktop OS should make use of large screen real estate and the precise targeting of the mouse cursor. The mobile OS should be optimized for the small screen and for the rough tap of the finger.

It doesn’t make any sense to port a user interface optimized for mobile touch devices to the desktop, and neither does it make sense to give tablet users the desktop interface. Each was custom built for its own environment, and each is optimized to be operated by different methods. In their compromise, what Microsoft are doing now is giving a tablet to people who want to buy a PC, and giving a PC to people who want to buy a tablet. Since there is no hybrid device that works great for everything, there is no point in compromising the experience by designing a hybrid UI.

This is the exact issue I've had with Windows 8 since I first learnt about the approach Microsoft was taking with its design.

Jakob Nielsen writes a thoughtful criticism of Windows 8 from a usability perspective, highlighting some of the issues the new design paradigms introduce. The points he raises mirror my main concerns. It's a long article, so Jakob's summary is handy:

Hidden features, reduced discoverability, cognitive overhead from dual environments, and reduced power from a single-window UI and low information density. Too bad.