I'm not exactly the right person to answer this as I'm very happy with the usefulness of our site. But I'm not sure about other users or potential users. So I'd like to break it down into several use cases:
These are folks you have some particular problem and want to find a solution via Google. Currently, a huge portion (~80%) of our traffic comes from search engines. This is up from about 20% when we came out of private beta. I attribute this to SEI excellent SEO. In fact, "we explicitly optimize for “drive-by” [users] who type a problem into a search engine". I think it's safe to say that we are doing a good job of catching those users, but are we answering their questions?
That's a tough thing to measure, unfortunately. One solution might be to conduct a self-evaluation. I'd like to do that, but I'm not sure what it would show us. (Let me know in the comments if you'd like be to organize it.) Another thing to look at is our Greatest Hits. Ideally, we'd have a panel of outside experts look at our site and let us know if we are helping or hurting the cause of distributing correct information about the Bible and it's interpretation. My sense is that we are helping, but my bias is through the roof.
In the days of USENET, we called these users "lurkers". Personally, I think this is a valid use case. Obviously, we'd like to convert these people to regular contributors. In fact, this is probably the crux of the question. Real numbers on reader are hard to obtain. One measure is that over the life of the site, we've had 121 people sign up for our newsletter. But 24 people have unsubscribed. There are many reasons to unsubscribe and many reasons a dissatisfied reader might keep a subscription. Making a (potentially) wild guess, I'd say that about 80% of our readers stick around. If you have access to our anonymous feedback page, you'll see that 70% of our feedback is positive.
I think this group is the mostlikely to be intimidated by the site. I recall when I was reading, but not participating on comp.lang.perl.misc that I was very intimidated. I still was even when I was active there. Same thing with Stack Overflow and Gardening.SE and Philosophy.SE. Even publishing my thoughts on my own blog is a little intimidating to me. This isn't necessarily a bad thing, but I have some ideas about making this a more welcoming site for readers to contribute a bit more if they wish. (I'm planning to write a meta post about it.)
Some people learn best by asking a lot of questions. Conveniently, we need questions to build content around. Lack of questions, especially from people who are just curious, is a real problem around here, which is why I asked the meta question in the first place. I do know that the ideal asker was once an avid reader. That's because asking questions is very difficult. Someone who has been around a while and seen what works (and what doesn't) has a leg up in being able to ask quality questions. We aren't the only site to struggle with strike a balance between quality content and welcoming users.
The way I read our marching orders, the site's benefactors value those of us who work to provide quality answers. I'm doubly grateful for those folks who have actually acquired a knowledge of Biblical languages, studied hermeneutics in a seminary or other institute of higher learning, read and are able to cite scholarship, and so on. In other words, real experts. Again, I'm biased, but I think we have a great core of people and I want to make sure we are happy about the site. Kazark's answer is hugely encouraging to me. But I don't think Ray's question was directly related to people who feel comfortable answering questions here. Hopefully, we will be bold enough to raise issues on meta or on chat, unlike users who are uncomfortable with the site already.
I don't really have one, except that we need to put more thought into whether we are doing a service to those who are ready to move beyond merely reading the site to start asking. I think we can strike a better balance between being a great community and having great content.