If you're interested in my two cents for getting a lot of members/activity, I always consider these two options to be the best:
>> Specific Target + Rival Forums: For one, (at least in my opinion), there are way too many forums that get started up for generic purposes. *picks one* gaming forums. with 1 forum for every game they think will be cool. end result: almost always failure. By targeting something more specific than general chit-chat, gaming, etc, and focusing on a specific thing (like a certain game (or whatever)) will produce much larger activity than a new start-up that tries to incorporate everything, including all the
cool hacks that vB~.org has. (not that I don't think they're cool -- I just don't think adding all of them to your forum will help much in the long run). Also, by getting more specific, you have less competing forums to deal with, making your site an easier choice for a user to arrive at. Trying to do compete with huge gaming sites usually doesn't work well, but if you can provide quality, the users will see that you are better than the other similar boards/sites.
>> Originality. The only specific example I can come up with right now is
www.gaiaonline.com -- check their stats. They obviously did something right. They have something that not every board has or can download and install on their board. HUGE plus for drawing people in.
Any thought?