I added a folk metal ballad, and almost half of the recommendations were the complete opposite (fast songs with growls), no matter what options I selected.

Anyway, I liked it. It might need some refinement and may not work for creating a playlist (at least for me), but it helped me discover some tracks that are exactly what I was looking for. Thanks!

If this is what you are looking for, consider trying Pandora radio. Its whole premise is built on playing songs similar to ones you’ve selected.