There are a lot of new features I like in Mountain Lion, but one I don’t: I can’t seem to place a screen resolution button in the menu bar any more. This was useful when I was using a projector or second display.
But the next best thing is a keyboard shortcut for the Displays system pref: just press Option and one of the brightness keys on your keyboard. This one has been around for a long time, but it bears repeating now.
Check out an old post of mine for even more you can do with the Option key!