I think the form factor, not simply the screen size, is its most unique feature. The phone is incredibly thin and light considering the size of the screen and face. It's also incredibly balanced and comfortable in the hand. The camera hump is actually a benefit when holding the phone and I find that I can easily rest my index finger on the hump next to the camera when talking on the phone. The overall feel of the soft touch materials is great, too.
I would say the form factor is the most unique feature.
Probably multimedia (4.3" screen, DLNA, MediaShare, pseudo-HDMI, lots of video codecs supported out of the box) and specifically the camera app (8MP photos, 720p recording, panoramic camera mode, slow/fast mo video modes, noise cancellation video modes)