I spent quite a bit of time thinking about the recommendation algorithm and (after falling into a death metal hole I can't seem to escape on Spotify) I came to the conclusion that their analysis of the audio content of the song is way too shallow. You listen to songs because they have a common harmonic structure, or rythm, but not necessarily the same spectrum. That's why a metal fan might dig a cover of Metallica by Iron Horse

https://www.youtube.com/watch?v=3c7bISLhVl8

Collaborative filtering seem to be a much bigger contributor to the recommendation right now than objective factors about a song itself.