If that's what they're creating, they're not worth $500 million. Let's do the math: Let's say 5 million uniques a month, X 10 pageviews per user(that's asking a lot too) = 50 million pageviews a month. Let's say $10 CPM (that's asking a lot too).
That's $500,000 a month or $6 million a year at best. Great for a 1-2 person team. Not great for a startup asking for $500 million valuation. The pageview business model depends on huge huge mass. Not curated, premium Q&A. Hence why Yahoo Answers get a ton of traffic: because of a bunch of bullshit questions being asked every minute. Quora is the anti-thesis of that.
I think your math is a little off. When the Huffington Post was acquired (for $315m), their traffic was 15m daily page views. Assuming Alexa's not too far wrong, Quora's traffic is about 20% of the Huffington Post's at the time of acquisition, so figure ~90m pageviews/month already.
But really, they're just getting started. Quora has been open to the general public for less than two years. I expect their traffic will be much more of the kind Wikipedia gets than the Huffington Post does: long tail, evergreen content that gets a ton of free traffic from search engines. For that, and for the browsing that they so wisely encourage, you don't need lots of bullshit questions. You need lots of good ones that will turn up in search results, plus plenty of answers they find satisfying.
And there's plenty of room to grow. Wikipedia gets 100x the traffic Quora does now. I don't think Quora will ever be that big, but the $500m valuation isn't obviously insane.
That's $500,000 a month or $6 million a year at best. Great for a 1-2 person team. Not great for a startup asking for $500 million valuation. The pageview business model depends on huge huge mass. Not curated, premium Q&A. Hence why Yahoo Answers get a ton of traffic: because of a bunch of bullshit questions being asked every minute. Quora is the anti-thesis of that.