The new UI features thinner text and flatter icons. It feels more similar to iOS 8.
Following Apple's acquisition of Beats, it's no surprise that there's a Beats Music Channel. You can signup for a free 14 day trial directly from the Apple TV.
You can also log into the iCloud Photos app with your Apple ID to view your photos and photostreams.
In addition, the update supports Family Sharing which allows up to six family members to browse and access each other's iTunes, iBooks, and App Store purchases and share photos, a family calendar, and their locations.
You can get the update from the Settings menu on the Apple TV or you can find direct download links here.