Interesting post on tagging systems; why they work well on LibraryThing, and poorly at Amazon.

Takeaways:
- People will tag, when it benefits themselves, but not if it only benefits others.
- You need a critical mass of tags to be useful to community and overwhelm spam-taggers and opinion-taggers.
- You generally want over 100 tags applied to a book to get both highly relevant and complete tags represented.

Currently 43 comments on this post. Our friends at Peerworks (Shel Kaphan's non-profit tagging project, was one of the first to respond.