And the impact from 3GS to 4 were obvious ones like the front camera, FaceTime etc
But I can't honestly think of anything groundbreaking or revolutionary that apple can do
Huge screen upgrade (retina), front facing camera, FaceTime, Totally different design, thinner, faster, better back camera, led flash, HDR photos, 1gz compared to 600, 720p video and the battery was bigger. IT was a HUGE upgrade.
also the first iPhone to the the second was a big upgrade too. going from edge to 3g service was the biggest and the design was different. The 3gs to the 4 was the biggest upgrade apple has ever done for the phone in my opinion.