I think the problem is that the semantic web is fundamentally very similar to the sort of 'logical ai' (Prolog, Cyc, etc) that is often blamed for the AI winter.

Eliezer made a fairly convincing argument that a lot of this is the result of clever marketing campaign a few decades ago (http://www.overcomingbias.com/2008/11/logical-or-conn.html)

Well, so far nobody's really addressed the known issues with self-supplied metadata (see http://www.well.com/~doctorow/metacrap.htm for one good and old roundup)...