Like Dray said, primarily the members. Perhaps that is down to the moderation which was on your list but throwing out members who aren't what you're looking for is only half the way there, because if you don't pull in members you want, you will not have any once you throw out those you don't.

Second though, site design. It has to look professional, crisp. Like a fine fine suit. Just like TalkFreelance here. Yes, perhaps the green would make it like a suit for a Lephrachaun - but a suit nonetheless! And a crisp on at that

Then of course content, you have to direct the conversation of your members, presuming you have the first two sorted, to the right conversations through Forum choices and perfect moderation of those choice forums.