W. Eliot Kimber - Topic Maps, Knowledge, and OpenCyc:

"I think that topic maps are useful and attractive as far as they go: for the general business problem of managing metadata and associating it with data objects, it's well suited and well thought out.

Why do I think that topic maps (and anything similar, such as RDF) is not suitable for knowledge representation?

For the simple reason that knowledge representation is much more sophisticated and subtle than just topics with associations. "