John Udell has an interesting, though short, article on del.icio.us and Flickr and their approach to classification:

Conventional wisdom holds that people will never assign metadata tags to content. It just isn't on the path of least resistance, the story goes, and those few who do step off the path succeed only in creating unwieldy taxonomies. (Do you file the revised XML Schema specification under xml/specifications or specifications/xml? We can never agree, and many good minds are sacrificed in the vain attempt.) Yet somehow, users of Flickr and del.icio.us do routinely tag content, and those tags open new dimensions of navigation and search. It's worth pondering how and why this works.

Abandoning taxonomy is the first ingredient of success. These systems just use bags of keywords that draw from --and extend --a flat namespace. In other words, you tag an item with a list of existing and/or new keywords. Of course, that idea's been around for decades, so what's special about Flickr and del.icio.us? Sometimes a difference in degree becomes a difference in kind. The degree to which these systems bind the assignment of tags to their use -- in a tight feedback loop -- is that kind of difference.