David Cole, product designer at Quora, writes:

The big, obvious change to the look in iOS 7 is the flatness. This change is being characterized as a stripping away of dimensionality. I’ll propose something else is going on here: the move to flat screens actually affords a ramp up in dimensionality. When an individual screen gets flattened together, you can treat it as a single object that you can then manipulate and relate to other screens. This concept is at the heart of the biggest changes to the iOS 7 interaction paradigms.