There is a new Q&A website, currently just called math.SE, for discussion of mathematics at any level. Part of the point is to provide a useful complement to MathOverflow at a less advanced level.
The website has just moved into public beta, and ideally will become a full-fledged StackExchange website in a few months. I’d encourage you to give it a try! It’s not quite at the same level as MathOverflow, admittedly, but even if that bothers you, you’ll at least get to see what SE 2.0 (which I’ve heard MO is going to migrate to) is like.