Daily updates and app syncing
OS X Mountain Lion centralizes all updates in the Mac App Store's Updates pane. When you do a Software Update from the Apple menu, the App Store opens and does the search -- not a separate app as before. That way, all your updates are in one place.
OS X Mountain Lion also now checks for app and system updates -- including security patches -- daily, rather than weekly. And you can no longer select that update schedule; it's daily or manual. OS X also now can be set to install security and system updates without asking (that's the default).
You can also have OS X automatically download any Mac apps purchased on another Mac using the same Apple ID -- just like how iOS works.