Great screen: you can’t see the pixels, thanks to the high-quality retina display (although there’s some debate regarding the resolution of the human eye).
Two cameras: this is a cute feature, and I can imagine FaceTime would be cool if I could just find a friend with an iPhone 4 to try it out on! I believe the 3GS may be able to view FaceTime calls, if not initiate them. Skype are also interested in having FaceTime capabilities in Skype (and so are many end users), but there’s been no progress that we’ve heard of recently.
Apart from that, maybe it’s faster, but I haven’t noticed any discernable difference with my old 3GS. There’s a gyroscope in there too, but I haven’t installed any of the fancy games that make use of it.
If you’re a 3GS owner, maybe you don’t need an iPhone 4 right now, unless you think the resolution is a must-have. You might as well wait for the next iPhone if you haven’t splashed out already…