I was doing some research for the ISMIR tag tutorial when I stumbled upon (via this interesting paper Playing Tag: An Analysis of Vocabulary Patterns and Relationships Within a Popular Music Folksonomy by Abbey E. Thompson):
The following expert from this paper:
[...] "tags are often ambiguous, overly personalised and inexact" [...] "The result is an uncontrolled and chaotic set of tagging terms that do not support searching as effectively as more controlled vocabularies do." [...]
This was published in the D-Lib magazine in early 2006. I wouldn't be surprised if by now the authors realized they were wrong.
But why would anyone ever want to control the vocabulary people use when describing something so extremely multifaceted and something that evolves so fast like the content on the web (delicious), or snapshots of life (flickr), or music (Last.fm)? I guess I'd need to think more like an old-skool librarian to understand that.