Radio. You can now start one on a song, an artist, an album or a playlist, and it keeps going for as long as you let it. It's in the ⋯ menu everywhere a song or a card appears, plus a Radio button on artist pages next to Shuffle. Starting a radio on the track that's already playing leaves it playing and replaces only what comes after it, so nothing re-buffers, and anything you added to the queue by hand survives. When YouTube has no mix for a track, the app asks which mix that track belongs to and uses that, instead of appearing to do nothing.
Home has a Familiar Artists section, built from the artists you actually play, with subscribe and shortcuts on each row. If you have already rearranged your home feed it slots in after "Jump back in" rather than sinking to the bottom.
The queue and lyrics panels now float over the page instead of pushing it aside. Opening both used to squeeze the content into a strip too narrow to read.