The updated version of KDE/Plasma in Kubuntu 16.04 changes a number of the default shortcuts compard to those in 14.04. One of the main one I'd muscle-memoried is Ctrl-Alt-Del to initiate a shutdown. There is a way to get this back in 16.04, just requires a couple of steps.
- Open the Global Keyboard Shortcuts dialog. Select "ksmserver" from the KDE component dropdown. Click the "Log Out" entry. Define a custom shortcut to whatever is preferred. Press Apply.
- Open the Desktop Session Login and Logout dialog. In the Default Leave Option section, check "Turn off computer".
Now the option to shutdown will be the default option.