Or: upgrading macOS
I held off on migrating because of my tablet driver (Wacom). Wacom sent all customers an email asking them to wait — the tablet might stop working. Why they didn't sort out the driver in advance I don't understand, and maybe I don't want to.
The upgrade was somewhat slower than a minor update. Effectively everything happened in the background and all I had to do was click and wait...
If I had to pick the key changes, it would be the "new" behaviour of the Play/Pause button. As soon as Spotify/iTunes is running and I have a Safari window open with a video, the button prioritises Safari. Which really annoys me — and I'm not alone. On the plus side, a keyboard shortcut for locking the screen has finally been added.
What did annoy me was that all my WiFi settings disappeared, so I had to set everything up again. Otherwise I'm happy with the new macOS version.