I could approach this from several angles, having some experience in the area, however I think one thing that hasn't been expressed here is making sure the bar is low enough for people to contribute good content.
of the many reasons the forum is successful two are 1) you have a core group of contributors who eagerly and expertly assist any newcomer, and 2) the youtube videos match the forum in terms of helpfulness, straightforwardness, and honesty.
The second is no fluke - people who are attracted to the videos will try the forum, those who are turned off by them won't. You are the example, and people appreciate your style and humility.
The first is related, but there's an important difference between this forum and some others - this one has a low hurdle to participation. Registration is required, which keeps the moderation requirements low, but that is the only bar to participation. There are a number of forums which I would have contributed to - including this one, at first - but didn't because I had to register. I'm biased, but I don't think I'm the kind of contributor that forums want to avoid. Expert analysis and good explanations are candy to search engines like google, and are exactly what newcomers are looking for when the search for information. Google doesn't just measure whether a search link is clicked - it also measures whether the one below it was clicked, indicating that the one above it didn't meet the user's needs. People who end up here via searches likely often get their questions answered, and then stick around if this is a topic which they are researching. Google sees this, and pushes this forum's results up.
That said, I won't participate in forums where I can't contribute fully. Locking posting features behind a pay wall, or even demanding a certain number of posts before allowing attachments or images are things which will turn me away and avoid the forum except in relevant google searches. I can't tell you the number of automotive forums for cars I own that I'd love to visit regularly, but due to this, too many ads, the requirement to login just to see images or download files, etc I just ignore.
So I'd strongly suggest avoiding participation limits. In fact, I prefer the stackoverflow model where registration isn't required - enter your name and email along with the post and let the members flag it and moderators delete it if it's inappropriate. If considering ads, limit them. - one at the top, one on the side, one at the bottom. Start putting them between the posts (or worse - making them match the posts in color and visual style, or having so much at the top that the content doesn't start until you scroll down) and I'm gone.
I prefer the clean visual look this forum currently has, so unless there's some reason to change things then keeping it as it is is preferable.
But I'm cheap and time constrained, so if the forum starts to make using it even a little difficult, or requires payment of some sort I'm just going to take it out of my daily rotation and only visit it if it happens to show up in a search. Losing me isn't an issue that should direct your decisions, but some people are easily annoyed yet have some value, so it's worth considering the balance of the low bar vs moderation vs expert participation when approaching these decisions.
