I cannot stress enough that the community is what makes the forum popular or not. Yes, this may sounds very 'DUH!'-ish like in a response. But what I mean is that you have to have the right kind of community, in order for complete strangers to feel welcomed and accepted.

This is probably the hardest part, of starting up a forum (because it works with advertising) because you've got to find people with all kinds of personalities that are willing to stick with you in the beginning towards the end.

Active community is a relative term. A site can make loads of general discussion posts every day, or have a lower rate of posts but are very focused on a topic and well built.

As you said it depends on how the site is 'marketed' to be found by those who are really interested in it. It''s a tough and neverending task, but as you grow bigger you can utilize members to help you as well. For example, asking their friends, posting in social networks, or leaving links at sites with similar interests.