I was going to recommend Quora too, it comes up a lot when I search for stuff, although honestly, I'm not sure how tightly they regulate who can answer.

There are also a few specialized subreddits, depending on what you want to ask about. I don't know all of them, you'd have to hunt them down, but the ones I've seen have pretty good moderation to keep people who don't know what they're talking about from answering.

Examples:
AskScience: Got Questions? Get Answers.
Medical Help Subreddit
AskHistorians
AskLegal: Justice for all.