1. listening to records with a friend 2. going to a dance club 3. attending a live performance 4. listening alone while reading liner notes 5. pawing through a stack of records at a flea market 6. reading a book
So for "discovery" to be a viable product I think it has to somehow be at least as engaging as any of these more traditional ways.
That's not to say that various curated playlists and algorithmic suggestions are useless. But it's rare that I'm introduced to something new that I like.
The algorithms are good at guessing that if I like the Zombies, I'll like the Turtles (which is true) but it never jumps to (for instance) current, contemporary indie/DIY stuff that is influenced by '60s rock and '70s punk. I make those associations all the time. Indie/college radio DJs do it all the time, but Spotify never does.
But again, Spotify is trying to sell millions of subscriptions for people who just want something to listen to in the car or put on at their dinner party. For the "hands-off" experience, it's fine. But discovery by its very nature is not hands-off.