What exactly is the site all about? I only ask because it doesn't seem to have a central theme or purpose that the site revolves around. From what I have personally seen, forums that are too "general" in what their purpose are seem to fail. It becomes very hard to get members to join and stay, when the mission statement is along the lines of "everything computer related!", or "games, music, and movies!".
Sure, sites can evolve into a more multi-purpose forum, but to get started, a site needs to focus in on a specific topic, and that will remain its anchor. Take CP for example. It has boards for entertainment, programming in general, computers and technology, general discussion, and more, but its mainstay is RPG Maker. In all reality, its pretty much RPG Maker XP, since this site seems to focus on it much more than VX, VXA, or any of the earlier ones. Its safe to say that everyone here has used, still uses, or is interested in RPG Maker. That is what gets them to join.
If you want to start a forum about programming, pick a language to be the "base", then allow the site to evolve and expand to other languages on its own. If its about books, appeal to a very specific genre of books, and allow it to evolve and expand into other genres. I'm sure you get the point, but as you can see, this allows you to have a solid foundation that will keep loyal and returning members.