I'd probably be fine still using my iPhone 3G were it not for the camera. An upgrade every 2-3 years feels practically mandatory for better photos.

Honestly the biggest thing I notice is the 5x zoom camera, everything else is a "wow that's nice" when I do a direct comparison, but I promptly forget about it (similar to looking at 4K HD vs 480i and then promptly forgetting about it when actually watching the movie; so many times I've realized it was using the older smaller file).

Battery life, I guess, if I had to pick something else.

Have they started becoming better again? I had the X before the current 13 and am still regularly disappointed by the camera.