I think it's the audience, the people participating. What.cd had a lot of very enthusiastic, competent, knowledgeable users while Spotify features a rather mainstream, general user base that takes a more passive role when it comes to music and its discovery. Obviously results will differ.

Furthermore, Spotify hardly provides any tools to promote and showcase a high quality, carefully compiled playlist. I can share a link, sure, but that's the most basic of paths to new potential listeners.