You pretty much nailed it. Quora was a fad for the valley's elite. It was launched with a critical mass of the right people that pretty much everyone in tech had reason to jump on board right away.

But if you grow this quickly you're also likely to implode this quickly, especially with the fundamental conflict of interest between startup vs community curation. As far as I'm concerned, the only way to make a great community is to prioritize that over profit and growth. That's why HN, MetaFilter, and countless smaller communities and boards can thrive long-term, but "community" startups like Digg, Reddit and Quora tend to go to shit. The closest I've seen to bucking the trend is Stack Overflow, but that's due to subject matter focus, phenomenal long-term utility, and abysmal competition, and even there it's hard to argue it's still as great as it was a couple years ago.