Clarity regarding controlled vocabularies, taxonomies, thesauri, ontologies, and metamodels. With all the scuttlebut going around about folksonomies and tagging, these are important terms to understand. In the process of tagging, it's pretty noticeable
C. Cattuto, D. Benz, A. Hotho, and G. Stumme. Proceedings of the 7th International Conference on The Semantic Web, page 615--631. Berlin, Heidelberg, Springer-Verlag, (2008)