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
L. Marinho, K. Buza, and L. Schmidt-Thieme. Proceedings of the International Semantic Web Conference, volume 5318 of Lecture Notes in Computer Science, page 261-276. Springer, (2008)