Let's start with a continuous brainstorm about the new Classification system of Contents we would like to see within Plone.

CMS Features

Taxonomies, facets, categories, classifications, keywords, tags, etc. are all ways to divide the big pile up into smaller piles.  Yup, that's what a CMS is for also.  What other ways does a CMS impose unique issues on classification?  Read zopepaul's writeup on CMS Features .

Here some pointer to what is just said/written in chronological order:

• Death and Taxonomies by Jonah Bossewitch
  
• Plone Taxonomy, Revisited and How Plone Keywords Should Work by Jon Stahl
• Product for simple content tagging by Jon Baldivieso
• A new Plone classification system (a Plip proposal) by Enzo Cesanelli
• the Collective Faceted Navigation product

A list of what other systems (cms or frameworks) just do it in the classification field or what they're doing better than Plone.
Let's learn from them!

Drupal (cms)

• Taxonomy: A way to organize your content
• Implementing Facets in Drupal

Faceted Classification and Browsing

• Facetag (project)
• Simile Exhibit (M.I.T. web application framework)
• iTunes (Apple application)

Semantic Web Resources

• MOAT (Meaning Of A Tag): a framework to inject meanings into tags