There are a lot of questions that can't be asked on StackOverflow, especially ones that don't necessarily have a right or wrong answer.
HN is more focused on sharing news and less on asking questions, especially potentially silly ones.
On Quora, you can ask basically anything. It's like Yahoo! Answers, but with a much better interface.
It's a lot easier to ask a question on Quora than a place like reddit because you don't have to worry about finding the "correct" subreddit. Make a mistake with the wrong subreddit and you'll just be instantly downvoted and get nothing but joke responses.
That said, the site does have a Sillicon Valley "I want to be rich" twist to it, and I think it's necessary for that tendency to die off for the site to succeed in the long run. Far too many times have I looked at Quora only to see a lot of threads about getting rich, what's it like being rich, what would you do it you got rich overnight, what's it like to date someone richer than you, how do I be rich without being arrogant, etc.