I remember when Pandora was attempting to compute the "DNA" of a song (assorted classifications like key, tempo, style) and recommend music based on that. You could then get a "station" based on a single song, and fine-tune it by adding more or disliking songs as they came up.

The end result was underwhelming- it never really captured the characteristics that I actually liked about particular songs, and ended up being crappy or so narrowly tuned that it lacked enough variety to be interesting.

The concept is still around, but with less scientific sounding fluff and, I think, more relaxed parameters for recommendations.