Users often come for one topic and leave. How do you expand their engagement into other areas? What design choices encourage exploration without overwhelming them?
I say answer their questions but also ask further questions to keep them engaged. You can also tag them in relevant other posts to get their views and replies on those posts but make sure they're relevant to the original posts they were discussing.
I agree with everyone's answers above, was literally going to write the same thing. I noticed that giveaways, especially for launching a forum, prove to be really beneficial. Whenever I launch a forum, I usually hold a giveaway that rewards active members. This drives users and drives content.
Another thing that easily throws me off when I visit a forum, if it is too complicated to navigate. The theme/software needs to be UX friendly, complicated or "busy" themes are a turn off and are not visually appealing. I'm a UI guy, if I see a beautiful interface and clean design, I usually stick around