Fast, high quality answers from very intelligent people.
Quora's community is small and is primarily composed of people in tech/academia (probably because of its invite system). Real names are displayed (profiles are often linked to facebook accounts) so any response carries some weight of reputation. Every user has a timeline of new questions and recent responses that are filtered towards topics that the user follows. Presumably, the users follow topics they are knowledgeable about.
So:
1. The invite system builds a small and intelligent community.
2. The filtered timeline and low volume of questions enables fast responses from users.
3. Along with #1, the addition of reputation yields high quality answers. The topic following system gets questions in front of "experts" efficiently.
4. There is a voting system for responses much like stackoverflow.
Quora reminds me of HN + Facebook profiles + Stackoverflow.
It's invite only. It is more useful than the others because of its high SNR. It's useful for members of the Silicon Valley echo chamber because it's entirely populated by members of the Silicon Valley echo chamber.
They seem to also have good usability and cool tech that helps keep thing even more relevant, but the challenge will come when it is opened to the "How is babby formed?" Yahoo Answers public.
http://ask.metafilter.com prevents eternal september by intentionally making the sign-up process slightly difficult. Rough interface and a $5 fee for new accounts.
And extremely active moderators who are themselves respected, accessible community members, and who will actually help new users understand why their posts or comments were deleted and how to act within the social norms.
The product just works so much better than the rest in terms of the social mechanics + natural flow of user interaction. You could have asked the same thing of what does dropbox do that box.net, xdrive, (insert 10 million others),etc. do better? Dropbox just works so much better that once you experience it, you'll certainly agree.
I was an early user of Vark and I loved their product in the early days, but once the floodgates opened, I think the way it was used didn't logically scale. I was getting lots of random ims + unrelated questions.
http://www.mahalo.com/
http://www.formspring.me
http://vark.com/
http://answers.yahoo.com/
http://www.chacha.com/
...