Screen brightness automatically adjusting in OS X Mavericks
There is an automatic brightness adjustment setting, but it's not new to Mavericks.  In System Preferences, check Displays; the setting is called "Automatically adjust brightness".



It adjusts based on a light sensor near your MacBook's built-in camera.  I've found it to be fairly good, but occasionally tipping my MacBook Air's display back a little further back than usual will make the light sensor detect more light and brightens the display considerably.