In addition to the design update, Microsoft also updated the apps for Windows and OS X to full release (they had been previews until now), will release an Android app in a few weeks, and added an API for developers to use any SkyDrive file in their apps. The rollout will take a few hours globally, so we haven't gotten hands-on with it yet, but we'll let you know if there's anything else tucked away in there once we do.
The new unified Microsoft experience is a drastic change, and not everyone's going to come along quietly. But whatever you think about these "modern design" changes, you can't argue they aren't pretty. Or at least, a hell of a lot prettier than what we had before. [Windows Steam Blog]