After A Few More Days With Apple's iPhone 4S...
At the hardware level, there are a handful of changes to the iPhone 4S that Apple doesn't even bother talking about. The display, for example, is improved. It includes better contrast, a more standard white temperature, and higher luminance.
The iPhone 4S' camera also demonstrates some notable gains, which are particularly identifiable in low-light conditions. And whereas we saw a bluish tint in pictures from the iPhone 4 (which made it look great outside with lots of light, but less so everywhere else), the 4S takes snapshots with better white balance.
Battery life can be less on the 4S, but not by much, and not necessarily in every situation. Compared to the previous generation, you'll only see a difference in processor- or graphics-intensive tasks, such as playing a video or running a taxing game like Infinity Blade.
Siri is more of a mixed bag. It's useful, but by no means smart. For the most basic questions and commands, it gives you the answers you need and performs the operations you ask of it. But it's far less impressive to us than Apple's hardware changes.