There's been some interesting conversation about the future of taxonomies in Plone over on the Plone Developers list lately.  In particular, Arnar Lundesgaard's responses to Martin Aspeli's smart questions are worth reading closely.

Martin's questions, as summaries by Arnar, were:

1. You'd like better categorization in Plone, not just folders and smart folders and were looking for opinions. You also mentioned some approaches in your mail to the list.

2. You wanted to know sort of categorization systems that "works in the real world", mentioning RDF triplets and tag clouds as examples of systems you have heard of.

3. Something that can be quickly implemented in Plone but that you do not have to rework for the next version of Plone.

Arnar's response is too long to reproduce in detail, but a couple of the less-technical bits that leapt out at me.

First, on the importance of user stories and usability:

First, you need to figure out what and how you would like to present the
functionality to the end-user before you select any technology.

Interaction design is the key phrase. Keep it simple. We learned early on that all the flexibility in the world does not matter if our users are unable to understand or use the features.

On the pros and cons of tags:

Advantages:

1. Tags capture your own context at the time of tagging.

2. Tagging makes you stop and think about what you are classifying.

3. Tags use your own words and perspectives of what is important.

None of these work as well when you are classifying for others:

1. Visitors do not share your context, experiences or view of the world.

(This is a problem for other categorization systems as well, but [particularly true given] the freeform nature of tagging.)

2. Folksonomies (ala del.icio.us) work because you can to a certain extent rely on crowd-wisdom. This may not be possible in other systems.

3. Tags tend to be generic, meaning lots of hits for each tag.
(This suggests that it is the intersection of tags which is interesting and a decent interfaces might overcome this. See Jon Udell.)

4. Synonyms, or multiple tags meaning the same. (Or do they?)

5. Tags may be problematic [on] multilingual websites.

6. Free-form, how do you manage the tags?

All of the problems with tagging or similar keyword based systems may be worked around, though I believe you simply end up creating an ontology language. (Often a poor replacement, though that does not mean it is without value).

While cool, tag clouds are just a visualization and perhaps not the best way of navigating a document collection. (Nor area they a well known pattern from a usability perspective)

Finally, even poor tags are better than nothing.

On making classification easy for end-users:

Classification carries a cognitive cost, meaning that it is work, even hard
work. You want your system to minimize the cost of doing this (what tagging
does so well). AJAX techniques *really* help here.

On the importance of understanding prior art before implementation:

I recommend studying literature on patterns and usage of existing technologies like RDF and Topic Maps before implementing your own classification system. I certainly found that very illuminating and it helped to clarify my own thinking.

Very smart people have thought about these problems for decades and repeating their experiments and mistakes might [be] time consuming.

Lars Marius Garshol's "Metadata? Thesauri? Taxonomies? Topic Maps!" at
http://www.ontopia.net/topicmaps/materials/tm-vs-thesauri.html is a good read.

Good stuff.