As detailed by Xbox spokesman Larry Hryb on the Major Nelson blog, the console's party app will now make chatting with your friends easier, thanks to the addition of new notifications that provide a better understanding of your connection and microphone setup.
The update also makes it easier to get back to a game hub page, thanks to added links within activity feed posts. And finally, What's On, a new section that provides links to popular media (be it games, movies, TV shows, etc.) has been added to console's feature set as well.
However, both voice messaging and dedicated server support for party chat are not included in this update, as Microsoft wants to spend a bit more time refining these features within the preview program.