"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Detailed Panther Info and Screenshots
Meanwhile, RailHeadDesign has posted an easy to follow and extensive screenshot-walkthrough of Panther. (Click on Various Page, then An OS X 10.3 Panther Visual Tour)
Added: FunMac has an ongoing thread regarding Panther.
Added: TheMacMind has posted another set of screenshots.