What percentage of questions would IBM’s Watson be able to answer on Quora?
Real world questions tend toward messy and rambling (like asking three questions in one, and more often than not ungrammatically, if not misspelled or even in SMS jargon), which makes it really difficult for question answering machines. Both WolframAlpha and TrueKnowledge have question answering APIs available today, but seem to only be able to answer brief factual questions. One of the problems with learning machines is that they tend to become degraded when released into the wild, because people will drag them down to the lowest common denominator. So in answer to your question, I would say Watson would be able to answer only a small percentage of questions on Quora.