Is Stack Exchange supposed to be just for experts, or mainly for experts, or is that up to the individual sites and not something really set by the overall StackExchange?
Well, here's a quote from Jeff from the launch of Stack Overflow back in 2008:
The idea that you have all these experts waiting in the wings to do stuff is an illusion in my experience. There's really just a bunch of amateurs muddling along trying to do things together. The people that are truly experts are too busy to even help, right? And if the experts are too busy to help, what difference does it really make if there are experts at all. Because the whole point of this endeavor is helping other developers, and whether you're an expert or not, if you have no time to help, you're not really contributing to the solution.
I think the frequent use of the word expert might imply "genius" but that's not what SE is about. SE is about contributing your expertise and helping out in things you know. It's a well known and understood concept that you cannot be an expert in everything and therefore sometimes need to ask - for example, I know my way around C, but give me a Ruby project and I'm lost at sea. Hence:
There's really just a bunch of amateurs
I think that's slightly tongue-in-cheek. It's less "we're all rubbish" and more "we all have a time in our life where we need to ask a question".
Taking that analogy out to all the SE sites, many sites look like they're getting a lot of expert advice left right and centre. That doesn't mean that only "experts" can participate - it means people with the experience and knowledge in domain of the question have answered. These same people may well be asking their own questions tomorrow, or the day after.
tl;dr Stack Exchange is for getting answers to questions about actual problems you face. Your expert-level "status" is, with a few notable exceptions, irrelevant if you can explain your problem or help somebody else solve theirs.
Edit, as I had to dig this up. One of my favourite blog entries anywhere on the internet is this: Does one have to be a genius to do Mathematics? - this is from Terry Tao, one of the few people in the world who could without a doubt be given the title of a genius. I'm going to quote his closing paragraph verbatim:
The objective in mathematics is not to obtain the highest ranking, the highest “score”, or the highest number of prizes and awards; instead, it is to increase understanding of mathematics (both for yourself, and for your colleagues and students), and to contribute to its development and applications. For these tasks, mathematics needs all the good people it can get.