What makes Stack Exchange different and unique from other Q&A sites? Why would someone use Stack Exchange over say Quora?
Quora is horizontal: one site, all fields of knowledge. Stack Exchange is a number of verticals: many sites, one field of knowledge each. That gives you a more sparse coverage of the whole knowledge shebang, but by virtue of being vertical it's bound to have an higher average reach than Quora does, providing you better content.
Disclaimer: I don't have a lot of experience with Quora, just what I can see from a bit of answering and browsing stuff. Please correct me if I am wrong about any of its features.
Most Stack Exchange sites favour specific, objectively answerable questions over broad, open-ended discussion-y ones. Mere opinion polls that seem to be accepted on Quora, are discouraged on SE.
Answers are expected to be equally precise, and to back up their claims with arguments and sources. Posting mere opinion as answers is discouraged. This is different from Quora, which seems to be encouraging more open-ended discussion and opinion. (Not saying that's a bad thing per se, but it's not what SE is about.)
Stack Exchange sites have - by design! - a very narrow focus, and extremely strict on-topicness rules. And indeed, the question of On-topicness is a major part of the sites' internal discussions - sometimes even serious clashes. Content that is deemed off-topic is migrated or deleted. (an archiving solution is said to be in the works but nothing has been forthcoming so far.)
Obviously, this narrowness makes Stack Exchange a bad fit for certain kinds of questions, especially those that require ideas, visions, experience, opinion, and speculation. If you have these kinds of question, you may run into trouble with the local community. I guess that's why some people love SE (I do), and some hate it.
Stack Exchange doesn't:
- Require you to create an account to participate (purely on the basis that the Quora home page doesn't show anything other than a Login / Sign Up prompt).
- Profess "to be the easiest place to write new content and share content from the web". Instead Stack Exchange provides Q&A sites and their associated communities which are targeted to specifics.
- Own your content. Neither does Quora, but if this question is generalised, it's worth mentioning.