Its the people. The active posters are real and its not difficult to figure out who they are. Some are published authorities on the subject. Many attend brewing functions like NHC or judging regionally, etc making it kind of foolish to be a douche when you might likely meet face to face. On the other hand if you do mess up you can just say sorry and get over it without ongoing ridicule. Another thing is maturity. Most of the active posters dont behave like 8th graders. I strive to behave at least like a 9th grader.
All of what Jim said, plus a really great group of moderators who know how to explain the rules to people in a manner that makes everyone feel like friends, and a membership who appreciates a friendly environment free of drama.
Inn the "old days" of the AHA, there was no forum. When I was elected to the Governing Committee 9 years ago, I started agitating to get one set up. So they said, "go ahead"! I had a pretty clear idea of the kind of place I wanted to hang out and I'm glad to find that there are so many others who feel the same way.
Please, invite whoever you want. Hopefully, they'll be attracted the the helpful, friendly, informative atmosphere here and contribute right along with everyone else.