Clearly everyone has a different opinion of what makes a great forum, a great forum. For example, when I joined what's forums I noticed it was full of elitist, worthless egos, and could find nothing more. I made probably less than 10 posts, because it didn't matter what I said, someone came along and disagreed for the sake of disagreeing. Call that a mature forum, I call it a 12 year old with an inferiority complex.
Don't get me wrong. I can surely not convey this through a wall of a text on a forum, but I'm one of the most out-going/understanding/non-judgmental individuals you'd meet, and I can take a good beating in an argument and still respect the other party. I just don't enjoy the beating if the person arguing with me has no idea about what they're talking about.
If you were to ask me, I define maturity by the thinking capability, and reasoning capacity of individuals. Not by how many dead bodies they've seen or how many lives they've saved. If someone seems to have a solid rationale, can easily convey his flow of thought, in a pseudo well-mannered way, I will respect said individual. That's kind of the reason why I liked ScT forums. Sure, you could say they were full of trolls - but they were the good kind. Nobody who ever had a legitimate intelligent question was flamed. It was the people who had stupid questions that are answered in a dozen other threads that got that treatment, and while people might not agree with me, and I know they won't, I think that's the height of maturity. Teaching someone to teach themselves, rather than spoon feeding a moron to death.