Here a few favorite settings for a newly installed Linux Mint 18 KDE. I divided the list in usability, security, and applications & adjustments. It makes no claim of being complete:).
First thing to do after installation is to install updates, and along with it configure the Update Manager. Under Edit → Preferences make sure that you check "Always select and trust security updates".
Desktops with different wallpapers
Especially Linux Mint with the KDE flavor can be customized to a high degree. In LM 17 I had several virtual desktops with different background images. In KDE Plasma 5 this does not function any longer. This is greatly missed by many users.
But there exists a workaround: click the "Plasma toolbox" button in the upper left corner, choose "Activities", and create a bunch of activities (instead of virtual desktops). Then add a wallpaper to each activity. With a right click on the panel choose Task Manager Settings → General, and uncheck "Show only tasks from the current activity". Now it's almost like working with different virtual desktops in LM 17.