Break it down further, you have the four cornerstones of Microsoft: Windows, Office, Xbox, and I'm going out on a limb as to say Bing/online division. ALL of those products have undergone a huge revamp in design this past year and most recently this summer. Windows 8 has been reimagined, Windows Phone 8 is built off Windows shared core of 8 making it a huge change again in Windows Phone, Office has gone through a cloud centric revamp and just a luscious UI, and Bing has been redesigned to be more immersive and more in line with everything else along with Office Web Apps, SkyDrive, ect. To top it off, all of Microsoft's products have a metro/mosaic tile design to them, especially Windows, Windows Phone and Xbox. The four squares, or better yet, tiles, make perfect sense.