What works for programmers doesn't necessarily work for other groups (and vice versa). I will be shocked if any of these becomes a site as important in their respective field as SO.
As for Quora... Please for God's sake TechCrunch will you shut up about Quora. It's a Q&A site with a Facebook login that is liked by the Valley insiders--and is thought important by the same--because the other Valley insiders use it.
Until it gets appeal beyond the Valley it's a storm in a teacup. What's more I'll go out on a limb and say Quora will never get mass appeal.
To the Quora founders: congratulations on the Emperor's new clothes. Sell up before the jog is up.