WordPress 3.9 lead developer Andrew Nacin has hinted there will be a secret feature in the upcoming release.
Now that features-as-plugins are the norm, core features are usually known about weeks before a new release. They are ever picked apart in great detail during public core development meetings.
But this time, Nacin has a surprise up his sleeve. During the latest #wordpress-dev IRC meeting meeting, Nacin casually mentioned there would be a secret feature, but didn’t elaborate further.