Peter Merholz at Adaptive Path continues the conversation about 'folksonomy or, 'metadata for the masses':

The primary benefit of free tagging is that we know the classification makes sense to users. It can also reveal terms that “experts” might have overlooked. “Cameraphone” and “moblog” are newborn words that are already among Flickr’s most popular; such adoption speed is unheard of in typical classifications. For a content creator who is uploading information into such a system, being able to freely list subjects, instead of choosing from a pre-approved “pick list,” makes tagging content much easier. This, in turn, makes it more likely that users will take time to classify their contributions.

Of course, it's not a perfect solution:

Clearly, such tagging systems are not a panacea; they present many potential drawbacks. With no one controlling the vocabulary, users develop multiple terms for identical concepts. For example, if you want to find all references to New York City on Del.icio.us, you’ll have to look through “nyc,” “newyork,” and “newyorkcity.”

You may also encounter the inverse problem — users employing the same term for disparate concepts. Flow, for instance, can either mean optimal creative experience, or the movement of a fluid.

But it doesn't really have to be. There are ways around the problems above and, as is clear from looking at Flickr and del.icio.us, they are powerful tools that can be more flexible and could certainly add more usability to traditional metadata systems that sort of work but never quite the way we want them to.