Put simply: They were more complete.

Spotify's library isn't even close to what WCD's was and is missing some copies of tracks/albums and/or just has the wrong versions flat out.

Can't agree with this more.

And the efforts of hundreds of thousands of engaged music fans who deleted bad tags, promoted good ones, made the connections.

It's frustrating sending fixes to Spotify when you come across something incorrect. It takes so long for any action to be taken that I've given up.

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.

Exclusivity. You had to earn the right to make a collage, and even then you only had a few so you really needed to put thought into it.