The changes to know about
The biggest change you'll notice is the new camera button. I'm still getting used to it after a few days, but I'm already defaulting to just pulling the phone out of my pocket, tapping the button and taking a picture.
My wife rightly asked me why I don't just hit the camera button on the lock screen like on earlier iPhones. I don't have a good answer for that. It just feels more natural to push a camera button.