It's all well and good to point out that there's a need for more answers, as the unanswered questions mountain is not getting any smaller (currently at almost 400 questions), but there's another important thing:
Vote early, vote often
Does your site have a healthy middle class of users with vote up and down, and edit tag privileges? Does it have a healthy governing class of users with edit, close, and moderation privileges?
Quite frankly, Health.SE doesn't. Sure, there's around 30 users who could vote to close questions, but most of the closing I see is due to a moderator closing a question. Rarely do 5 users vote to close one question. I see 4 closed questions on the main page right now, all of the closed ones were closed by a moderator, with between 1 and 3 user votes.
Out of the 10 users with most rep, 6 haven't written an answer in over a month and 2 haven't even logged in during the last week. Looking at other users with more than 500 rep, I see more of the same pattern.
There's an average of 6 questions coming in every day. Even considering one or two get closed, the number of unanswered questions gets higher. The problem, in my opinion, isn't only that there's not enough users with high reputation, the primary problem is that there's not enough answers, and a secondary problem is that there's a lack of users voting*. And this leads to this problem of not enough active users with moderation privileges. On a site that only has two moderators at the moment, that's problematic.
I'm afraid I can't really give a solution to that problem, just share my thoughts on it, and this is way too long for a comment.
- more voting could lead to more motivation. If a new or newish user answers a question and doesn't get the 2 or 3 votes they usually get in a day here (my experience), but instead gets 5 or more, that might lead to more motivation to invest the time for more answers
- getting faster answers might lead to those asking questions getting more involved, maybe even starting to answer. If a person asks a question and gets no answer or gets an answer months later, they often don't seem to come back
As it is, I think the site is in quite a bad state :-/
(* Looking at the list of voters, there's only 9 users who voted more than 10 times this month. I don't think that's a particularly high number.)