How can I keep my Mountain Lion MacBook Pro display at maximum brightness?
When you go from mains power to battery, OS X decreases display brightness. (It should go back to normal brightness when you reconnect to mains power.)

To turn this off, go to System Preferences > Energy Saver, choose the Battery tab, and un-check "Slightly dim the display when using this power source".

The display can also change brightness because it senses a change in ambient brightness. To turn this off, go to System Preferences > Displays, find the window on the display you want to change this setting on, go to the Displays tab, and un-check "Automatically adjust brightness".