I'm sorry Frogboy, for all the great work you have and still will put in to the game, you have very little influence over the community.
It's a confluence of the anonymity of internet communication and the justification people feel in their oppinions. People are hard-wired to be agressive towards viewpoints they are opposed to, and will defend both their own views and also to the game, which they also feel invested in.
Some people will be nice, some will be not-so-nice, and some will simply be trolls. You can probably convince people to be nice for a while, but unless you sticky this and force people to read it before they're allowed to post any reply ever, it's going to be meaningless over time.
You could assign a Community Manager to supervise a mod team to keep things civil and welcoming in here, but that's something that has to be kept up over time for each new offender. Some companies (I believe SoE?) have taken the rather draconian measure of linking forum behaviour to the game (forum ban = game ban). That's really pretty much all you could do.
Maybe the community can be shaped, as you're getting to it early... but I really doubt it. This is a spectator sport i'm afraid, make the best of it.
SOE's forums are a complete joke. Banning people for bad mouthing your game or voicing actual problems and concerns is never the answer. Complaints, issues, and concerns are all healthy things that need to be brought up in order to make things better. Yes, some people will troll, that'll happen in every forum, but just because people say something you don't want to hear doesn't mean you should silence them. It's all a part of pushing the game to be better.
SOE out right bans people who say ANYTHING even remotely negative about their games. The result of this is EQ2flames has more forum traffic than SOE's own forums. People know they can go to EQ2flames and talk all the shit they want, they can say ANYTHING they want and no one will be banned for it, you won't even be moderated. Yes, there are a lot of trolls there but there is also a level of discussion about the game that official forums could never hope to come close to. Since discussion is the sole reason for forums a more laid back, free speech approach fosters that far better than the Nazi approach.
Now, as far as the DG goes, you have a competitive game (pissy people), revolving around a team based system (pissy people), on the internet (anonymity, pissy people), with the potential of a very large audience (crowd mentality, pissy people). There are going to be problems based on any one of those things. Combine them all and you expound the problem. It's just the nature of the beast. People belittle the DotA community for being the way they are but I promise you this, this community will be exactly the same, and it's on its way there. It's really has nothing to do with anything other than the nature of the beast.
