As with any message board, the Admin. can only provide a visually pleasing, easy to use environment- The rest is up to the participants. In my opinion the reason this site is successful is mostly because of the cross-section of people that choose to not only come here but more importantly to participate in the discussions here- That's the whole key to it.
As far as being free, I have not come across one message board out there yet that is not free. So that is a mute point, really.
I believe you asked that question mainly because you are trying to get your own boards going, and lately the going has been disappointingly slow at your site. I feel for you. The saying "If you build it, they will come" does not apply to message boards- Message boards only get off the ground if in the beginning many people make a dedicated effort to get the boards primed, and continue to do so over several months if that's what it takes- A good example of that is over at above10000.com. A quick note- The hyperlink to your site has two r's in the word "forums" and so it's a dead link the way you have it. Maybe that was intentional... I don't know. Good luck.