Designed by Pentagram, the preeminent design studio. Surprised I didn't find out about this earlier. My take is... it's boring, but really what alternative is there?
The window points away from the wordmark instead of towards it, which is distracting for me. Actually I'm indifferent about the design, it's the self-fellating design talk that gets me. "The team designed the system to fit into lines of perspective." – Four squares is a "system"? "The clean lines, flat shape and bold color of the logo reflect the design principles of Microsoft’s Metro design language." Clean lines, flat shape and bold color, therefore it's awesome! Haven't heard that one before. If only they used Gotham, it would have been a winner for sure.
From the windows team blog: "Our final goal was for the new logo to be humble, yet confident. Welcoming you in with a slight tilt in perspective and when you change your color, the logo changes to reflect you. It is a “Personal” Computer after all." I don't think the logo is very humble or confident. It's more precocious and stoic, a distant but intimate embrace of hue and shape, altering its nature to mirror the viewer's physical spirit.
Seriously though, the logo is pretty interesting because I get the feeling of a big shift at Microsoft. Whether or not this is reality is something else, but looking at the Metro "design language", it seems that Microsoft is actually trying something different with the aim of improving their product.
Pentagram's Windows 8 page
Windows Team blog