I just pre-ordered my iPhone 4S yesterday. I currently have the iPhone 4 so I'm definitely not in need of an upgrade, but the new features in the iPhone 4S were just too good to pass up. The biggest feature that I am excited about it the Siri voice assistant. We've had voice dialing and simple voice commands for quite some time now but it has always been awkward to use and required you speak in a strange robotic voice. Now with Siri, no more!! You can simply speak to the phone in a natural voice using natural language. In addition to doing things like looking up weather, scheduling appointments, and calling people, Siri can also read text and emails to you as well as respond to them. If you haven't already, you should definitely head on over to the Apple website and check out the video on Siri.
The other two standout features of the iPhone 4S for me are the upgraded camera (8MP and larger aperture) and the dual antennae (doubles the download speed currently available to the iPhone 4 on AT&T's network and improves call quality). My new phone arrives on Friday and I can't wait!