The camera, of course. Every iPhone after the 3G has shipped with a higher-resolution camera than its predecessor. People capture and share a lot of photos on their iPhones, so a very likely culprit for higher data usage, controlling for OS version and tethering abilities, is that the photos are simply much larger with each new iPhone.
The camera on the iPhone 4S is a significant upgrade in terms of quality and performance over the iPhone 4. Great photos mean more iPhone 4S users will take more photos. And it would be reasonable to assume more photos taken also means more photos are shared, consuming more data. If the camera gets better in the next iPhone, which is quite likely to happen, expect even more data consumption.