I think in this day and age you have to appeal to very core needs and strong desires of users, you have to do it well, and you can't do it in a wishy washy way.
What I mean is this. People might come to a forum because you have some unique and interesting posts there. But really that's super wishy washy.. people can get information for free at their fingertips on any topic day and night.. easy. Why use your forum when they can use other places.
What they can't get as easily might be a) ways to tangibly make money, and b) people to talk to who could be (i) practically useful to talk to or (ii) just fun to be sociable with.
You need to either provide clear, easy to understand, and easy to execute ways of generating money, that can only be achieved using your community, or you need to have people there already that are very active and that people want to be sociable with.
Both of these things are rare commodities, most other reasons to use a community have only minor pulling power.
Critical mass is the key thing though. It takes money to make money, and it takes people to get people. If you can't fund the initial people you need, don't expect anyone else to want to waste their time there either.
And I think something that is often overlooked is your communications with members. Having a strategy for autonomously or semi-autonomously contacting members regularly keeps the community in their mind.
There's so much going on that it's easy to forget about a single website if you're not reminded. By creating automatic notifications, newsletters and reminders each day or each week, it keeps members feeling connected. If you don't do this it's just too easy for massive numbers of members to "drop off" and forget.
