With Windows 8, Microsoft is rethinking its desktop OS. Explorer, the file manager, is one of the key components, as it represents one of the most used parts of the system. Microsoft approached the design of this element with a data informed approach, based on telemetry data recorded from real users (and this means millions of sessions).
The file manager is not one of the sexiest parts of an OS, but it is a very core one. Microsoft's approach with it is quite conservative: win8 Explorer is not a transition to something new. It implements solid ribbon concepts, in a different context.
Be sure to check out the interesting post about it that was featured on win8 development blog.