Wordpress 3.9 Release Candidate 1 is now available for download and testing.What’s new in this RC?
TinyMCE received a major update, to version 4.0. Any editor plugins written for TinyMCE 3.x might require some updates.
WordPress 3.9 now uses the MySQLi Improved extension for sites running PHP 5.5. Any plugins that made direct calls to mysql_* functions will experience some problems on these sites.
Other new features in wodpress 3.9
live widget previews and the new theme installer
UI refinements when editing images and when working with media in the editor
advanced display settings for images
links will appear in the media manager once you’ve uploaded an audio or video file
added HTML5 caption support to match the new gallery support
The formatting function that turns straight quotes into smart quotes underwent some changes to drastically speed it up