Hmm hidden...
There is a GUI for plugging in a hardware security key?
I want to use mine for unlocking KeepassDX, which doesnt even work, but yay there is a GUI!
You can display whatever app you want on the lockscreen, the app just needs to request that. For example "secure photo viewer" is great when giving cops your phone.
You can use ADB and grant some apps some cool permissions, so they can do crazy stuff like change the systems energy settings.
3rd party apps can add as many shortcuts as they want to the quicksettings and their long press menu. This is really cool.
You can permanently backup your contacts as .vcf