App preview clustering control
Mission Control is one of those features users puzzle over, then one day get it. It lets you see all your open windows and spaces (collections of apps to help you organize a cluttered work environment) and navigate among them. It debuted in OS X Lion and shows the power of a touch interface, though it works well with a mouse, too.
In OS X Mountain Lion, there's one subtle but nice enhancement: In its system preference, you can have the Mission Control view show document windows for each app grouped with their app or have them presented independently of each other.