A good idea (and others offered plenty already) is the starting point but even a great idea will fail without one or more friendly, professional, knowledgeable users who are on a lot at first. Any new forum can get a lot of fly by. There has to be a reason to stay; site-stickiness. Eye-candy or 2.0 features is never one in the end. The quality and availability of the community always is. So it takes a couple of founding users a lot of time and work at first. Until you get a critical mass. Here for example there is a currently active core of something like 100 "expert" users. That's pretty amazing actually.