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.