Old-school iOS: hyper-detailed icons, drop shadows everywhere.
iOS 7 and later: clean icons, layers and "3D" depth and responsiveness.
Say what you want about "flat" vs. "skeuomorphic." The fundamental driver behind much of the iOS 7 changes is the new virtual lighting. Instead of simulating light coming from above the user's head, iOS 7 simulates light radiating from inside the iOS device itself. Few if any drop shadows. Much more translucency. More "truth," in that light really is radiating from the iPhone's screen.
iOS and its devices are far more important to Apple now and in the future than OS X and Macs. Because of that, we'll see design trickle down from iOS to OS X and iCloud. Better to have a strong family resemblance between all software products. Less confusion for users, more Apple branding throughout all products and services.
And, as a side benefit, iOS 7 and OS X will look completely different than "the competition." Android, Windows Phone, Windows 8, Windows RT, and whatever other OSes there are out there will all look old-school compared to iOS and OS X. It might take a year for Google to warm over Android to look as fresh and new as iOS 7. It might take Microsoft 2 years to warm over Windows Phone to look fresh. And it might take Microsoft 5 years to warm over Windows desktop to look fresh. And in 5 years, there may not even be a Surface. So there may never be a new, fresh-looking Windows RT.