Dmitry Fadeyev makes the best case so far for why it’s not a good idea to combine mobile and desktop operating systems into a unified experience, like Windows 8 has done. From Blurring of the Lines:

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. The desktop OS should focus on power users and multi-tasking, the mobile OS should focus on content consumption. The environments they run on are different, the use cases are different, and the solutions should be different.

That’s exactly right. This “unified experience” sounds like a decision made from the viewpoint of devices and technology, not use cases. For example, if you make decisions based on devices and technology, you may decide to create an iPhone app before you know what kind of phones people will use your service on. If you make decisions based on real use cases, you may actually find that very few people would use your service on a mobile device, so a better solution would be to (gasp!) optimise for desktop use¹.

The irony is that even though Microsoft made a huge deal about their “no compromise” design philosophy, the Windows 8 experience will have to make compromises if the same software needs to work on both mobile and desktop devices.