I simply do not see #4 happening. Hell, at this rate I'm willing to bet that StackExchange, itself a rather niche site (or network of sites) has higher activity than Quora.
I don't know a single person outside the (somewhat incestuous) startup industry that uses Quora. In fact, I do not know a single person outside the tech industry who knows about Quora.
Claims that Quora has achieved good traction are, to be polite, overly generous, but more frankly, bullshit.
They have a reasonable diversity of backgrounds (it feels about on par with Reddit or MeFi in that regard) which leads me to think it's not entirely a tech industry phenom.
Also: they have an interesting strategy of courting mainstream media distribution; there's a "Quora" section on Slate now, for instance. They could keep doing that.
9 figure valuation? No clue. Valley numbers, agreed. Just saying.
Slate is more mainstream than HN, but not mainstream. Forbes is a better example of mainstream reach, but also the risk: the Forbes audience is not a quality content crowd, it's an infotainment filler crowd, and infotainment is cheap these days.
Yahoo Answers tried to be the One Q&A Site to Rule Them All. I don't personally consider Quora to be a general purpose Q&A site.
Its possible that they launched as a general purpose Q&A site and then they may have switched when it seemed prudent. Or its possible that they may still be trying to be a general purpose Q&A site, but in that case their branding doesnt match their ambition.
Having #1 and #2 are good for attracting the proverbial greater fools to take it off your hands. Also add #5 - use and/or answers by people the main stream public has heard of. Some more of those, particularly celebrities, politicians, entertainers saying sh*t, could have this become so mainstream that their answers will be quoted on the Nightly News.
So it becomes a complain and explain platform like twitter.
2. Founder ponying up millions of his own cash.
3. Frighteningly Ambitious(TM) problem.
4. Reasonably good traction and product execution.