I can’t give you a detailed answer since I just signed up for this site. But I would say that the best Q&A site I’ve ever been on was something called WHquestion. It was shut down, years ago now, and I’ve been looking for a site that can replace (or at least come close to replacing) it and so far, this one seems like a pretty good site.
For me, for a site like this to be considered good, it has to have more intellectual types than just people looking to screw around on the internet. People who are truly interested in what they ask and in turn, people who are truly interested in answering questions.
Not only that, but the layout and general design of the site matters, too. It has to be appealing and have good features that are going to draw people in. When people create sites, you can tell if they’re truly interested in the purpose based on how much time is put into creating it. Anyway… That’s all I can say for now and I’m eager to explore the site a little more, so I’m off! :)