Since we're only one year into our iPhone 3Gs, there's probably very little motivation for me to try to do an early upgrade.
Digital compass? Eh, fun but not earth-shattering.
FM radio/transmitter? Would be nice, but there are many apps that stream FM stations, or just other media in general, and as for the transmitter, I've got Bluetooth with A2DP, so it's working out well even in the car with my Motorola T505.
Higher megapixel camera? Nice, but again, is it worth the early upgrade costs and efforts?
More capacity? 32GB is nice, but I have a hard time filling up 16GB as it is, even with a bunch of movies. It's also very time consuming to watch it sync up that much stuff.
More battery life? I'm speculating on this one, but I'm hearing the next-gen iPhone will retain the same shape, and I just bought a mophie Juice Air Pack, so I'll be OK still.
Video recording? If this was only in the next-gen iPhone, it's a slight possibility. I've lived without it for two years, and I've dealt with workarounds such as my own trusty digital camera.
Front-facing camera? We haven't heard anything on this front, but if there's any indication we could have videoconferencing via iChat, Skype, or some other program in the App Store, I would upgrade both me and my wife the same week.