Actually I feel like Google is the one such service that is approaching natural language usability for general topic queries. It seems to me to be able to extract basic meaning from simple English phrases and search that sentiment. Whereas search phrases containing negatives ("not", "isn't") used to be futile, Google now correctly infers your meaning.

But yes, for parametric search I think I would always prefer a direct interface. It makes clear what is indexed and what isn't.