The Metro design philosophy is the defunct Zune team's true legacy. Their work on the PMP went to waste mostly due to timing, but their design work has lived on in everything else Microsoft has done since. Virtually all the core tenants of Windows Phone and Windows 8's design, from typography as hierarchy to live tiles and colorful flat design, were first seen in the Zune desktop software and Zune HD's UI.
I wouldn't consider Metro game changing. The design and principles behind it was floating around in Houston long before I saw it appear on Windows devices and the Xbox console.
Microsoft made it popular but it's been around for a while in response to design driven web designer vs info and interface driven design.