Much better screen, much better camera, and waterproof. It's a little faster, but it's not a huge difference (I mean, the S4 was pretty fast already). But I'm so much happier with the camera and the screen is a big difference, especially outdoors.
I may be in the minority, but I love the fingerprint scanner. I don't know why, but I hate entering PINs or swipe codes, and it's way better than face unlock!