Previously, Xbox One users could watch Twitch streams, but not stream their own gameplay due to the lack of an upload application. With this release it adds the uploading ability. The level of integration on the Xbox One console is said to be quite deep.
Perhaps the most interesting feature however, is the ability to actually jump into games that are being streamed live. In addition voice support has been added to allow the use of voice commands to start broadcasts, archive game clips, and view other streams.
The streaming of gameplay from the new next generation consoles seems to be one thing that many who have already purchased a new next generation console and those looking purchasing a new next generation console are evaluating as of the most important new features that this generation of consoles offers. It remains to be seen however if this is the killer application that is going to sell more consoles.