You're the Pundit: What's the killer Mountain Lion feature

When it comes to forecasting the next big thing, we turn to our secret weapon: the TUAW braintrust. We put the question to you and let you have your go at it. Today's topic is Mountain Lion.
This week, a developer preview was released and Apple updated its site with a list of upcoming features. They range from an iOS-inspired Notification Center to Game Center support to integrated Twitter.
Which of these is the killer feature that will help define the new platform and set it apart from earlier operating systems?
You tell us. Place your vote in this poll and then join in the comments with all your predictions.