Much has been said and asked about the differences between Stack Overflow and Quora.
And, while there are deep and interesting differences, such as how Stack Overflow makes reputation tracking and badges explicit, in my opinion, one simple difference is the most important of all: Quora's login-wall.
See, you cannot do anything with Quora until you've registered, while with Stack Overflow you can do almost everything without registering. They are polar opposites!
Like everyone else, I have too much curiosity and too little time. I try to keep up on Hacker News (sorry Digg and Reddit): I click through to the cool stuff, and then move on. You have one precious first page impression to rope me in, so don't spend that impression with a login-wall!
I mean, sure, I'm still going to go link up my Facebook account so I can login to Quora and see the questions, answers, conversations. (And, yes, Facebook seems to be winning at the "universal ID" game, even though I like OpenID better.) Still, for each persistent user like me, you've lost 9 non-persistent ones with that dreaded login-wall.
Remember: if you are are a new cool Web site, gaining value from the network effect (as all social sites do), trying to eek out just a tiny slice of all these fickle users jumping around out here, don't put up a login-wall! It's just about as bad as a paywall. Let brand new users do as much as possible with your site, and make that very first page impression count.