System settings is missing

You can try to reinstall gnome-control-center from terminal:

sudo apt-get update
sudo apt-get install --reinstall gnome-control-center

In case you install Gnome 3.12 on Ubuntu 14.04 via the gnome3-staging ppa you need to:

sudo apt-get install gnome-settings-daemon-schemas 

This will install the settings daemon correctly. There seems to be an issue with dependencies and correct upgrade paths.