I think that is a tough question to answer.

I just started a forum in Feb 2010 in an area where there are quite a few other forums. But I had been part of a community on a different board that was sold to a big corporation. For a few years I had thought about starting my own forum, because before the sale we had an absentee owner. People knew who I was, I recruited a few key people to help me get things set up and as my sounding board.
Here are my numbers from today:
508 Members
82 Forums
3588 Topics
50391 Posts

I have a nice group of members and mod group that are making things pretty good for a new forum. We are actually showing up on the first page on some searches.

I would say good content is important. Next IMHO is fostering the feel of community, people like to belong to something. Make new members feel welcome. When the forum opened to the public I started making a point if the new member posted in the Intro forum to post a Welcome, Glad to joined post. I still try to keep up with that. Once your membership grows, other members will jump on and do the same thing you are doing, make people feel welcome. I think you have to have a bit more than an interest, I think it helps if it is more of a passion.

What can you offer that the other forums don't? Is it your expertise in an area, Is it your experience in an area, Can you offer a more friendly atmosphere.

How much time are you willing to put into it to get it up and running.

Val