17 of 26Image
#16 It watches your eyes
The Galaxy S4 will track your eyes. This opens up the door to all sorts of interesting applications, like rotation tracking, where it knows what angle you're reading the phone at. I do a lot of reading in bed, with my head down and at an angle, and I've always had to engage the rotation lock to keep the screen from getting all wonky. This solves that problem.
At a more novelty level, the phone will also stay on as long as you're looking at it, will scroll web pages when you look down on the page, and pause video playback if you look away (which I'm thinking may lead to all sorts of psychological disorders if used too much).
While these are certainly not make-or-break features, they're part of what make the Galaxy S4 seem modern and current and the iPhone 5 seem so terribly dated.
Image courtesy my Galaxy S4.