The simple answer is that the key to Quora is its centralized repository of knowledge (and the way that knowledge is surfaced through social signals). 

There are plenty of Q&A plugins. Formspring has a way to post to your website, and Tumblr has an “Ask” feature. But aside from very popular people with very popular blogs, this doesn’t do much to get knowledge from brains onto the web in a meaningful way. 

The fact of the matter is that the most popular people don’t always have the best answers to things. In fact, you’d have to believe that more often than not, they don’t. Which is not stated but is one of the big issues with Scoble’s rant.

If Quora is about surfacing the world’s information that exists in people’s heads, there needs to be a centralized place to hold that data. At least until web search and structured data improves in that specific area.