Nested facets
A structural element of classification systems that I have never seen discussed explicitly has been hovering in my mind for years, and suddenly became kinda clear to me, so I'm writing this down ;) Sometimes really simple things can take a long time to sink in with me.
Many times, you'll have an overall taxonomy (say, "products") (whatever structure it has, faceted or not), and then, once you reach a certain point within that taxonomy (say, "digital cameras"), you suddenly are offered additional taxonomies to refine your choice (say, "lens type"). These additional taxonomies aren't just nodes within the higher taxonomy, they are additional facets, NOT part of the taxonomy you were browsing so far.
Try browsing epinions.com as an example, or have a look at this diagram:
These kind of nested facets are clearly being used and useful in real life, but I haven't seen their structural or other properties being discussed explicitly anywhere. They are not part of XFML, for example, nor are they part of Travis' Facetmap, afaik.
Maybe they are too obvious to be discussed, but some awareness of the existence and usefulness of nested facets (I'll just call them that, seems to work) would mean there would be more chance of them being built into the standards and tools that define the restrictions within which we often work. So who has worked with nested facets? Any tools that explicitly support them? Any comments on their properties?