The biggest, all but certain, feature of the next iPhone will be LTE 4G, because the new iPad has it. That's the main reason I'm sticking with the iPhone 4, not even considering the 4S. LTE is going to give you much faster data, faster than most DSL and some cable modems, if you happen to be someplace that has 4G service. It's also going to burn through that monthly data cap a lot faster.
Other than that, you can pretty much count on a faster processor, faster graphics, more memory and an improved camera. I would be surprised if there's anything as big in the next iPhone as Siri was in the current one.
Wish me luck and no regrets..... not more than 30 minutes ago I went ahead and ordered the iPone 4S... it will be here Tuesday.
I got to try out the Siri on my daughters 4s tonight.... I think that is going to work very well for me.