The W3C's Web Ontology Language is one to keep an eye on, a language to express classifications to enable the Semantic Web. Two problems I can see. Firstly, the maximum map size issue -- there's a sweet spot in terms of the maintainability, depth and breadth of classification systems, and I don't believe it scales to the dimensions of the www. Secondly, we've been wrestling with the ontology problem for thousands of years, since Aristotle, so I'd be inclined to label it as a Hard Problem. Or in other words, one that is unlikely to be solved by the W3C using XML (although I'd like to be proved wrong). An article that draws these strands together: From Aristotle to the 'semantic web'.