I just wrote up the first really big examination of book tagging on
the two largest book-taggers, Amazon and LibraryThing. I think it
might be interesting here.

http://www.librarything.com/thingology/2007/02/when-tags-works-and-when-they-dont.php

I conclude that LibraryThing has ten times as many book tags as
Amazon, and speculate about what this means in light of a much wider
disparity in traffic. I originally wrote it to include PennTags, but
it got out of hand and it didn't seem fair to compare them. But my
comments about how "numbers matter" in tagging is, I think, pretty
important for the future of efforts like PennTags, Blyberg's SOPAC,
the Swedish library and others. Tags only work in big numbers. (That's
why LibraryThing will be coming out with a "tag consortium" in the
near future.)

Tim