I think the fact that you don't allow downvoting posts contributes to this. I get that you want to keep things positive. But when you don't provide an outlet for people who want to simply disagree without forcing them to come out of the woodwork and defend their position, it is inevitable that some people will come out with a lot of ugliness. I think there are a whole lot of reasons internet forums are different than…
Users can downvote posts, but only users with karma above a certain threshold. No user can downvote a submission , though - you can only flag them.
Also karma doesn't count for a lot here (that being one exception). Ultimately, a good karma system should simulate the what happens in the best real world environments....where there is a very complex feedback loop that reinforces the sort of behavior we want and suppresses that which we don't want. In my opinion Slashdot comes kind of close, but it has huge flaws itself.