Nice article, HierarchyVersusFacetsVersusTags which gives a kind of information architecture / human factors view of hierarchies, facets, and tags — but it still seems to be that facets and tags are almost the same thing.

I just did a quick read, but I think the article argues that in a rich system, multiple shallow hierarchies are in addition to a tagging system, but a forced deep hierarchy, or only tags, are not as good.

See also:

• HierarchyPapers
• Ontology is Overrated: Categories, Links, and Tags
• The Flattening of Almost Everything #2: Information Retrieval … “People don’t think hierarchically … We think in terms of associations“

Also of note is the acronym LATCH, which stands for common ways of organizing information:

• L: Location
• A: Alphabetical
• T: Time
• C: Category
• H: Hierarchy