iOS has undergone a ton of small changes over the last six years, but never we have we seen as drastic changes as Apple has made with iOS 7. Don’t think iOS 7 is that big of change? Take a look at the image above that shows the evolution of the iOS home screen.
Jailbreakers have been able to customize their phones to look similar to iOS 7 now, but this is the first time that Apple is breaking away from some of the UI design principles that have made iOS so successful. The high res version can be viewed here.