I remember hearing the reason behind that in one of the BUILD conference talks this year. Apparently (and sadly) there are tons of high-profile Windows apps tha

I remember hearing the reason behind that in one of the BUILD conference talks this year. Apparently (and sadly) there are tons of high-profile Windows apps that would completely break and stop working all-together if the only thing they did in the next version of Windows was bump the major version number up one and changed nothing else. Developers hard-coded many portions of their product to depend on specific version numbers existing (instead of looking for available APIs). They learned their lesson after Vista, which is why 7 only bumped the minor version number up, and 8 will follow in its footsteps.