Looks like YouTube's been completely redesigned for TV sets. The visual refresh looks not unlike YouTube's iOS and Android apps, complete with a pretty, scrollable sidebar and a special focus on playlists.
If you're on the XBOX One, you should be updated already. If you have another set-top box, you should get the update shortly. If you're too broke to buy a big-screen TV, you can still check out the new design in your browser on your computer. [YouTube]