The new feature is the added support for in-video links (see above image). These are links to other YouTube videos that are placed over the top of the playing video via the annotations function on the web version of YouTube. These had been completely ignored until now on the Xbox One app which now seems to pull the linked videos' URLs and creates new thumbnails for them that are clickable during the last ten seconds or so of playback.
The new in-video links can be hidden or reshown by pressing up and down on the D-pad or left joystick.
Something else that's changed in the app is the search function which has been completely redesigned with a new vertical list of suggested searches that pull related searches from a user's history from not only the Xbox One app but also other versions of the YouTube app on iOS, Android, and web.
A full app update doesn't seem to have taken place to enable these changes so it's likely that some changes were made to the code remotely. Have you noticed any other changes to the YouTube app on Xbox One? Let us know in the comments below.