Snark, arrogance, responses that come flying at you. Mainstream people don't want to post on Quora. They're intimidated. One of the insecurities of dumb people is to have their dumb-ness pointed out. In their philosophy to ensure quality answers, they've driven away a lot of potential users.
There are some really intelligent, insightful answers on Quora. There's also a lot of attitude.
The sad thing is, Quora is a wonderful niche site. But since they've taken VC money and there are so many expectations in the startup world, they've put themselves in a tough situation in regards to growth. Though they should probably stick to the 'intelligent professional' niche, they probably need to pickup more mainstream users to justify valuations; but it's hard to go mainstream now with the userbase they've built up.