(yahoo_answers || answers_dot_com) + follow == fail
Quora has so many respected tech industry people using it that I want to use it. That's similar to how Facebook was first Harvard only, then Ivy League, etc. - you want to be where your perceived social betters are.
My Quora feed shows the topics that my friends are interested in. This is super valuable for finding information slightly outside what I would search for myself.
Their email notifications keep me clicking back to the site so that I stay engaged.
Once that happens, and it will have to happen for them to have the number of users that twitter or facebook has it will descend into yahoo answer hell. I don't see how they avoid that and get mass user adoption.