It looks like there's some good changes and not as good changes, just like every iOS release.
Some things I've noticed on my iPhone SE:
Tweaked app opening/closing animation.
New (old) signal bars.
Home Screen doesn't show in App Switcher. Makes it easier to close apps.
Can't activate iCloud Messages yet; the switch is greyed out.
Weirdly large-lettered titles in some apps. Like in Messages the top of the page says
Messages in huge letters. I know I'm in Messages, that's why I opened the Messages app

. Same for the
Settings app and
Bookmarks in Safari.
New notification center will take getting used to. I feel like it should be blurred.
There is no volume UI.
Calculator app is redesigned with round buttons. Icon is changed also.
There's an Emergency SOS tab in Settings. It lets you toggle between auto-call or slide-to-call when you click the lock button 5 times.
The new App Store is... not amazing. The entire UI feels like it's zoomed in a bit too far, especially the Today page. Also, there is no way to search through purchased apps.
Safari now scrolls like all the other iOS apps! No more swiping through molasses yay!
There's a shutdown button below Reset in Settings. Interesting.
Control Center. ...Um wat? I kinda get what they're going for, but for goodness' sake make it more symmetrical.
I can only customize the bottom row of icons, nothing else. It's a start though.
You can long-press on Control Center buttons if you don't have 3D-touch.
Screen recording is accessible from Control Center and is super cool.
Button Shapes in Accessibility now underlines button text instead of making that big grey outline thing.
New Smart Invert colors option in Settings attempts to *not* invert colors of photos, but does turn my dock blue.
Files app is useless for me right now 'cause I have no files to file.
Music is the same as it's been. I use Cesium instead.