Faceted metadata is very interesting. The notion of dynamic taxonomies and adaptive search criteria is very important for news databases where topics and attributes change much more rapidly than general search engines. For Moreover this is a crucial area.
I am less sure about XFML led by Peter Van Dijk, where the serialization of topics and facets seems to only allow 2 levels of hierarchy i.e. don’t see how you can have subfacets of facets or subtopics of topics. Now it is true that you don’t stricly need hierarchy beyond facet -> topics, but you get bloat, rather like flat file databases. Someone correct me if I am wrong.