I haven't got up to thing 8 yet but never mind. I thought I'd comment on Emma's post with some quotes summarizing the main pros and cons of tagging, but it would have been a bit of an oversized comment. Blog post in that case!
From the red corner:
Probably the major flaw of current folksonomy systems – and the number one gripe for those happier with more formal classification systems – is that the tagging terms used in those systems are imprecise. It is the users of a folksonomy system who add the tags, which means that the tags are often ambiguous, overly personalised and inexact. Many folksonomy sites only allow single-word metadata, resulting in many useless compound terms; the majority of tags are generally believed to be "single-use"; that is, to appear only once in the database of tags. At present there is little or no synonym (different word, same meaning) or homonym (same word, different meaning) control. The system administrators do not impose judgement about the tags chosen by users. Plural and singular forms, conjugated words and compound words may be used, as well as specialised tags and "nonsense" tags designed as unique markers that are shared between a group of friends or co-workers. The result is an uncontrolled and chaotic set of tagging terms that do not support searching as effectively as more controlled vocabularies do.
(Guy, M. & Tonkin, E., 2006. Folksonomies: tidying up tags? D-Lib Magazine, 12(1). Available at http://www.dlib.org/dlib/january06/guy/01guy.html)
From the blue corner:
(i) While the controlled vocabulary issues discussed above may hamper findability, browsing the system and its interlinked related tag sets is wonderful for finding things unexpectedly in a general area. In researching this paper, exploring the bookmarks tagged with “folksonomy” on Delicious, there were many recent resources from a wide variety of authors and sites that I likely would never have been exposed to. [although often the same is true of browsing using classification schemes or subject headings - ed.]
There is a fundamental difference in the activities of browsing to find interesting content, as opposed to direct searching to find relevant documents in a query. It is similar to the difference between exploring a problem space to formulate questions, as opposed to actually looking for answers to specifically formulated questions. Information seeking behavior varies based on context. While one could evaluate a folksonomy in a system like Delicious or Flickr by using specific queries from users, and then evaluating which documents tagged with keywords they choose are relevant to the query, that would ignore the broader set of browsing activities that the system seems to be stronger in.
(ii) Perhaps the most important strength of a folksonomy is that it directly reflects the vocabulary of users. In an information retrieval system, there are at least two, and possibly many more vocabularies present (Buckland, 1999). These could include that of the user of the system, the designer of the system, the author of the material, the creators of the classification scheme; translating between these vocabularies is often a difficult and defining issue in information systems. As discussed earlier, a folksonomy represents a fundamental shift in that it is derived not from professionals or content creators, but from the users of information and documents. In this way, it directly reflects their choices in diction, terminology, and precision.
(Mathes, A., 2004. Folksonomies – cooperative classification and communication through shared metadata, Graduate School of Library and Information Science, University of Illinois Urbana-Champaign. Available at http://www.adammathes.com/academic/computer-mediated-communication/folksonomies.html)
As far as I'm concerned, tagging and "traditional" approaches are both imperfect, and they complement each other.