The forums are really not that clean. In fact, most of the forums I visit are a lot nicer. These forums have sort of become undiscussable lately, since everything reaches 5 pages in a day, most of it just idle conversation.
DF hardly filters out the trolls, there are certainly plenty here. A troll is not born out of stupidity, it is born of a mixture of boredom and anti-social tendencies. Anyone who understands DF has to have plenty of time on their hands and tend to be just a bit sadistic.
Toady does do an excellent job of moderating the forums, though.
what forums are you visiting?
I'm looking at the general discussion, and seeing civil discussions about politics, books, anime, protests, security, various organizations, and so forth. I've seen long, civil, and productive conversations on religion, finance, gender differences, cultural differences, time zones, and a huge host of other things. The creative projects section is full of people making games, learnign programming, drawing pictures, writing poetry, and all sorts of crazy stuff. Life advice has practically written persuasive and informative essays.
The various forums on DF itself are generally funny and polite, and often springboard into discussions on research in the game itself. At other tiems it leads to long, great reads on what happens to other people.
What forums can top that?