I've been using Windows 8.1 beta for a while, and certainly brings a lot of improvements to the metro interface and the integration with the desktop that really makes it feel much more as one.

For example, the metro control panel controls virtually everything the desktop control panel does now.

But minorly, most importantly to me it allows the start screen be transparent over the desktop (though it hides everything on the desktop apart from the background). While, this seems really minor at first, it really makes the OS feel much more like one UI and that the start screen and the rest of the metro is an extension of the desktop and not a completely separate ui.

There are plenty of other good improvements, but I'm sure you can read about them online. Honestly, Windows 8.1 is the version of Windows 8 that should have come out in the first place.