Included in this release candidates are new camera support including the Phase One IQ260 and Canon EOS 70D, new lens profiles including those for the GoPro 3 camera. New features include a Smoothness adjustment slider to the Detail Panel to reduce low-frequency color mottling artifacts. Refinements to the Spot Healing tool and Local Adjustment Brush. Camer RAw get a couple of more additions including interactive Hisotgram like Lightroom, separate Auto Temperature and Auto Tint controls, and workflow presets are now available and can be selected on context-clicking the workflow link. You can read more about the big fixes and features for Lightroom or Camera Raw.
Release candidates are well tested beta versions that Adobe would like input from a larger community on its performance. Any feedback should be made on the Adobe feedback portal.