TouchID, A7/M7, Space Grey colour scheme (not a "feature" per se, but w/e), 120 FPS recording, Siri, and basically everything else about the phone.
For people who moved from iPhone 4S or 5, there aren't many huge features other than that, but since I came from an iPod Touch 4G, even some of the basic features of the iPhone that everyone takes for granted, like not crashing from Safari every two minutes, are absolutely awesome for me.
Been using it for a few months now, and I still haven't gotten over the wow factor.
TouchID is the biggest for me. It lets me keep a really complicated password for both my Apple ID and my phone, without having to type it in every time.