1) It to be relevant to my interests.
If I sign up, it's because I'm interested in discussing the subject.
2) nicely designed.
MLK has held me for so long because of just how simple and easy to use while just looking nice, it is...
3) A decent community.
This means you need to be moderating the boards well, if I see a ton of posts that just make me roll my eyes, I'm not going to join even IF i'm interested. Without a like minded community who all get a long well, it's just not the same at all.
I don't really know, just do the best job you can of making the forum as good as it can be, if you are passionate enough about it then you will probably figure most of this our for yourself eventually
