One area I really want to see this take off in is Medicine. As someone who had suffered from an unknown illness (no doctor could figure it out), I can rationalize how such a system would have been helpful. You see a bit of this with WebMD's Symptom Checker, but I feel tools like that aren't comprehensive enough and we end up with a lot of cyberchondria. You can't rely on co-relation to find absolute answers, but help…
In the medical domain, there do exist large structured knowledge bases and "expert systems" for diagnosis. Read up on DXplain[1], MYCIN[2] and the UMLS[3]. Even in biology, there seems to be significant activity in formalizing knowledge. It literally took decades to develop and refine these knowledge bases.
Creating something general purpose like the Cyc or Semantic Web is very challenging, especially because different people have different notion of "meaning". Just look at the back and forth arguments over some controversial Wikipedia page. This is 100 times more conceptual and challenging.
1. http://dxplain.org/dxp/dxp.pl