The other highlight feature lets Mixer streamers share access to their controller with viewers via an on-screen gamepad or by plugging a controller into their own PC -- we assume its low-latency streams come in handy here. A few other tweaks mean Xbox One streamers no longer come to a full stop while switching games or going to the dash, and they can start streams at any time.
The built-in Microsoft Edge browser is apparently more controller-friendly (also it can upload or download files now), and there's an option to change the balance between in-game audio and background music in the guide so you can substitute your own Spotify, SoundCloud or MP3-based soundtracks. Clubs can filter invitation requests to keep riffraff out based on criteria like Gamerscore for a particular game, and it's easier to join tournaments.
Separately, Major Nelson also noted three new security features on the Xbox One that make it easier to recover your account if someone has taken it over maliciously, find an account name or update your account recovery info from the console itself.
The first 1804.180227-1801 preview alpha is already going out and there's a list of known issues here.