Originally tweeted out by Arielle Vaniderstine, a "developer advocate engineer" at Spotify, Nelson is very much a version one release on Glitch. It's an internal project that feels more like a toy than an exact tool or proper feature. Still, it's a blast to goof around with, and the playlists it creates appear right in your Spotify app, if you choose to do so. I clicked "Ambient," "Disco," "Electronic" and "Happy" and got a playlist that started with Wild Cherry's "Play That Funky Music." It's a happy disco song, for sure, but sometimes the results are a bit puzzling.
The different variables you can tweak are explained at the Spotify developer blog. There are things like "acousticness," "danceability," "instrumentalness, "loudness" and "speechiness." There's even a measure for the amount of happiness conveyed by a track, called "valence." You can change these values in Nelson with a slider and the resulting playlist (which will play in your browser by default) should match your selections.