I've created throwaway accounts a few times to see if there's anything sufficiently worthwhile there. I'm largely convinced that 1) there's not and 2) the dynamics of the site work against it scaling or surviving.
So I'm not going to waste my time with it.
http://ycombinator.com/newsguidelines.html
Regarding Quora: its behavior runs counter to good netizen behavior. Clearly, people have an issue with that, and among those people is Paul Graham (as several others have cited and referenced in the discussions over the past few days).
Regards who you can encounter on Quora: in my experience the level and quality of conversation reached on a clueful but open site tends to be generally higher. You'll find quite expert people on HN (I've run across Charlie Stross a few times), and I've been online long enough to have seen numerous highly-qualified participants on Usenet, Slashdot, reddit, and even G+ (there are still numerous techies there). President Obama is among those who've run reddit AMAs, and I'll routinely find very qualified people within various subreddits there.
And again: the conversations are visible to whomever wants to view them without restriction.
It's not the registration, it's the morality.
They were certainly walking a very fine line, but Google seemed to give them a pass with this system for quite some time.
Google penalized sites that redirected from SERPs or buried content below a lot of "sign up here" stuff very heavily; you can even make the case that it could have been called the "EE Penalty" instead of Panda, if only because Google's web search team (i.e. Matt Cutts) collaborated heavily with the Stack ownership in developing the algorithm changes and consequences.
That's not to excuse EE's management's behavior -- quite the contrary. From the perspective of longtime users, EE's string of decisions, the consequences of those decisions, and its reactions to those consequences starting not quite a decade ago nearly destroyed what had been a vibrant community.
If there's anything good to have come of it, it's that EE has finally moved -- about six or seven months ago -- to a business model that allows the non-member to see what members see: the entire question and solutions. Joining for free does have a few minor advantages; paying (either with a credit card or by answering questions) has more.
But it took a long time for EE to learn those lessons and begin to implement those fixes. Whether Quora will learn them is a whole 'nother story; like so many other sites, it built its systems and market-share without much thought given to how it was going to monetize.
EE made that mistake too, in 1997. Quora has a lot of money backing it up, so it can maintain its facade for a long time... but I wouldn't be placing any bets on it being around 15 years from now if it actually had to depend on income.
The whole benefit of online discussion in general is to create as wide and persistent an archive of both questions and answers as possible. To that extent, I applaud StackExchange, even with its aggressive moderation (not too different from better-managed subreddits such as /r/askhistorians or /r/askscience): the audience isn't the person asking the question but all people who might want to know the answer to a given question. And they're better served by having a canonical response.
Quora blocking (explicitly) TIA means that that knowledge stays locked in their silo. Pretty much forever. Fuck that.
Just yesterday I was musing on energy (something I do a lot) and the fact that even in such advanced energy systems as nuclear fission and fusion powerplants (real or theoretical), all this advanced energy, provided directly in the form of electron volts, is degraded to heat, used to create steam, and then to spin turbines. That's fundamentally a technology that's hundreds of years old (and I think early steam or heat turbines go back further). Windmills date from the 1400s.
So I checked AskScience on reddit, and it turns out the question's been asked a few times. And I could read the responses. And, while, yes, I've got an account there, it's not necessary for that purpose:
http://www.reddit.com/r/askscience/search?q=electricity+nucl...