A lot of people have problems getting members, and a few of the problems are as follows.
If you have it set so members have to register and log in before they can view the forum, they’re less likely to register. The reasoning behind this is simple, you want to see if it’s worth participating in, you want to see what you’ll be talking about and who with. You may want to lurk around to get a feel for it before registering.
For valid comparison, SomethingAwful is a forum running vBulletin, and it’s pretty big. People can see a lot of the posts, and they’ve got about 80,000 members (I haven’t checked in a while, so I’m not sure), but the amazing thing about this is that because people can see the discussions that go on, they feel like being a member…
…and pay. That’s right, with as many members as SA has, it’s a completely paid forum. You’ve got to pay to be a member. If you had to register to view it, I would say it wouldn’t be nearly as big. (I do not suggest you even try to charge money to post at your forums, unless you’re not hosted by us)
Some other things that hinder users are things as simple as too many or too few forums. I’m not posting somewhere that has a thousand forums with zero posts and one or two active ones, in the exact same way that I’m not posting at one with three or four active forums. You’ve got to have enough to keep things split up a bit, but few enough that it doesn’t seem like you’re over doing things. As members request them, you will surely wish to consider adding them.
That’s all I’ve got for now though. 