Although Quora and Stack Overflow work in different domains, both inherently share one common thing, they are Q & A sites.
One feature that Quora has, which I believe is not on Stack Overflow (or I was not able to find it) is Ask To Answer, inviting someone to answer a question.
I am not talking about bounties, but a general request to someone to answer a question, that just might have gone ignored by people (as it happened to me of late). A person might not be willing to offer a bounty, in which case, their question remains unanswered.