The inconvenience of finding/creating a Facebook account to use in joining Quora is, imo, completely worth the value offered by Quora. You have to choose your battles. At a certain point I would rather have access than have my way.
I never bothered to sign up at stackoverflow because of OpenID. Not necessarily because I have some philosophical issue with it, but because I already have a system to maintain this stuff and OpenID is just another hoop to jump through. Adding an optional user/password signup is hardly difficult.
Nothing is stopping me from setting my password to something different for every site I don't want to set a good password for, I'd just very very much rather just enter a URL and have everything work. OpenID is the most useful thing online in ages.
The difference with Stack Overflow is that you can, and may people do, use it without an OpenID. Just go ahead and post a question or answer, and you'll be assigned a cookie-based account. And if you put in an email address, you can download a new cookie for that account at any time.
The problem is, in order to know that Quora is good enough to force you to use FB Connect even though you don't want to, you have to use FB Connect... I'm certain this slows their growth.
Or maybe it makes sure that the only people who go the length to sign up are aware of the benefit of Quora, and therefore likely to be quality users contributing the the quality of Quora. In a sense, Quora uses Facebook as a filter to get great users.
