Upgraded to 14.10, brightness control quit working - Intel HD 4400
Open a terminal with Ctrl+Alt+T and write:
gksudo gedit /usr/share/X11/xorg.conf.d/20-intel.conf
Type in your password, then copy-paste the following text into gedit:
Section "Device"
Identifier "card0"
Driver "intel"
Option "Backlight" "intel_backlight"
BusID "PCI:0:2:0"
EndSection
and save again!