iPhone Camera Comparison Pictures

The most immediate improvement I noticed with the iPhone 4S was the camera quality and speed.  Here's a nice comparison from the creators of Camera+ app showing pictures from all 5 of the iPhones.

And here's the slide from the actual iPhone 4S announcement that shows the speed improvements.  In fact, multiple sources have reported that Apple engineers began working on the speed improvements immediately after the previous iPhone shipped.  Camera speed and quality was seen as a major selling point and engineers were told that every 1/10th of a second mattered.