In Pictures: A sneak peek at Apple's upcoming OS X...
Instead of forcing Mac software developers to devise their own notifications capabiltiies, Mountain Lion features integrated, standardized notification technology. All alerts and notifications will appear in the top-right corner of the screen. Notifications remain there for five seconds, then slide off screen; alerts, on the other hand, remain on-screen until you click a button (Show, Close, or Snooze). That's essentially the same as how notifications work on iOS devices, though the Snooze option is (so far) unique to OS X Mountain Lion.
In Pictures: A sneak peek at Apple's upcoming OS X Mountain Lion