What types of things interest you so much that you'll return to a message board?
I see we're having people post a couple of times and then not return. How do we get them to come back?
Accept it's not for everybody.
Personally I like to see a basic framework of rules that are clear but not intrusive. A board I used for a while but drifted away from (let's call it CF, for the sake of a name) had so many areas with so many rules it seemed like a relentless minefield trying to figure out where I was allowed to post. A different board I used (which I'll call BF, for the sake of a name) had that sorted out much better. It had a men-only area, a women-only area, and a young-people-only area. And that was it, aside from those restrictions if you were a Christian and a member in good standing (which meant little more than having 50 posts and a positive reputation) you could post anywhere.
Rules that do things like prohibit a married member from offering advice in the "engaged" area are stupid - there probably aren't too many married people who weren't once engaged, and most married people know a thing or two about being single, courting, being engaged and being married. Personally I find the presence of endless denomination-specific subforums counterproductive - the question "what do Catholics say Hesitations 4:13 means?" seems to miss the mark because perhaps the Baptists interpretation of that particular verse is the correct one. If there really must be endless boards that people aren't allowed to post in, hide them so the posts don't show up at all.
Some people want a busy board and get bored if there are just a handful of members who obviously know each other. Some people want a quiet board and will leave if there are thousands of people. Some like debate even if only for its own sake, others will shy away from a board that encourages what they think is pointless debate.
Some people will leave a board they believe promotes heresy, or allows heresy to be promoted. "Heresy" in this context is whatever the user in question thinks is heretic.