I think a better camera is a given, as every new phone model has a better camera in some way. So I don't think that will be the key differentiator for the new model. All the hype over the fingerprint sensor is almost so big that it has to be likely (and fits with the whole "S" theory, though there have only been two "S"s so far, and two is a coincidence, three is a pattern, so we'll see). I think there is a decent possibility for NFC based payment too. The public (i.e. stores) haven't really jumped on that wagon yet, but the same was true for Passbook and we're seeing more and more companies utilizing that. NFC would make sense as it seems like in a few years it will be very common, so giving vendors a year or two to "catch up" and allow that form of payment to be used would follow the same progression of the Passbook apps. Also, with Keychain being added to iOS 7, I think that lends some credibility to the fact they may be moving in that direction.
I don't think there will be any huge OS differences between the i5 and i5S that will cause people to say "I need this new device," but there may be a few small ones.