A good question and as usual, KK answers with enough relevant material to fill up a few Novels. To me there are a few key elements to a "Good Forum"
1. The forum itself must be well setup, well maintained, updated and of course be totally relevant to the individuals tastes and needs and most of all, run by experienced, competent and fair team of staff members.
2. I've said it before and I''l say it again. The best forum in the world is nothing without good members. By "good" I mean people who at least try to post productively and who can get along with their fellow peers at least most of the time. And also they try to truly contribute to the forums best interests. Even if in only a small way.
3. No Flaming, I don't mean petty squabbling. I mean hard flaming that's both hurtful and demeaning to a member. I should know, my forum had "Inter site/forum" wars for a longtime. I never liked it, not one little bit. But I tried to cater to all the members who had a point to prove. Faming destroys communities! Never allow it. Debating, now that's healthy.
4. Lastly. Don't join a forum if you're not going to participate. One has only to look at any large forums to see all of the "0" posters. About 4 years ago at TRMK. Out of roughly 2500 members there were 300-400 people who never posted. These included Socks, spammers who never spammed, lol, bored people, bots and the list goes on and on, dude.
But ultimately, in the end, it's what you really like in a forum that matters. It's a personal thing. You may post alot at a forum that has only 100 active members. I've been on forums that have as many as 50,000 members and yet they are practically dead. By the same token I've been at this movie forum for a year now andhave only 240 posts, yet I am there each and everyday, I just don't post much but I love the whole atmosphere there.