"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Apple TV 2.4 Update Now Available, Updated Remote App with Gestures [Updated]
One particularly notable feature however appears to be support for the newest version of Apple's Remote app [Free] which adds touch screen gesture support for controlling your Apple TV.
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Apple TV's menu options can now be navigated with simple flick gestures. Similarly Video and Audio have new flick gestures that can be used to advance, rewind and replay. See the screenshots above.
Update: Apple has updated its support document with details of the features in Apple TV Software Update 2.4:
Remote app directional control: Control your Apple TV with simple finger gestures via the Remote app. This feature requires Remote 1.3 running on an iPhone or iPod touch with iPhone OS 3.0.
Flickr Search: Search Flickr photo tags to find recently updated photos of interest. Save searches to quickly find new photos or use as a screen saver for Apple TV.
New view options: View movies By Genre, By Movie, or Unwatched. View TV Shows and Podcasts By Date, By Show, or Unwatched.
Updated transport and chapter modes: During video playback, click right or left to fast forward or rewind. Additional clicks increase the speed. Click down to show chapter markers.