I read an interesting piece by NPR the other day regarding this new graphical revolution to flatter design seen here on the iOS 7 screens.
I then was sent this by a good friend and found it to be even more interesting:
https://medium.com/design-ux/a93647e5a44b
Essentialy the tldr is a flat or empty graphic requires more brain processing as we subconsciously scan the inner and outer space around an icon, for example, rather than just the outside perimeter of a "non flat" icon that may have shading, gradient, etc.