Ugh, the Minecraft forums used to be actually halfway decent. Players might get into occassional arguments, but nothing too bad. And then someone linked it on Penny Arcade...then Something Awful....then 4chan. Griefing went through the roof, and now I can't even look at the forums for info. Blech.
Ummmmm, anyways, yeah, probably because the worst people from places like those can't take the learning cliff.
I hate to break this to you, but 4chan knows where bay12games lives.
Thing is, 4chan
likes Dwarf Fortress. Kitten genocide, mermaid factories, and magma cannons are kind of up their ally.
It's the same thing with TV Tropes as opposed to Encyclopedia Dramatica. Encyclopedia Dramatica somehow managed to raise the ire of 4chan, so it was Eternal Septembered into 4chan's stomping ground - the 4channers who wanted to remake the place in their image managed to flood the entire wiki with enough force that everyone who used to be there fled. TV Tropes, however, is something they actually like, and enjoy actually using as intended, or even editing in their favorite shows.
Beyond that, there's the Eternal September effect up here, too. The upper forums are "nice" because people apply pressure to follow the quasi-unwritten rules. When you act outside the rules, troll, or act abusive to others, there are those who are willing to either state what the rules are, or push back against the ones who keep trying. If you aren't talking about the proper topics for a forum, the whole thread gets moved or deleted.
It's not like it's hard to set up an account and start trolling, you don't have to prove you can play the game to actually get an account and troll. It's just that self-policing makes this forum a less juicy target for trolling.
Also, this forum has a unique form of annoyance. It gets some pretty hilarious sycophants that will attack any remotely similar game and worry about other games syphoning off Toady's revenue. See my sig for reference. "Goblin Camp" drove people loony for a while on this board.
That's one of the real lingering problem on these forums... Some people get so caught up in hearing positives about this game, and enjoying the game that they lose sight of reality. The people who point out bugs, have suggestions, or have complaints about the game in general are often subject to abuse by those who seem to think of the game or Toady as utterly infallible at times.
Aside from that, there's the occasional person who gives people using graphics packs a hard time, saying that, "if the game is not ASCII, this game will turn into Minecraft, and the forums will become 4chan." It appears sometimes in places like the Lazy Newb Pack thread.
Either way, it's people being too defensive about a misperceived notion of what makes DF a great game. Even if it is related to the self-policing that keeps the trolls away, it's like having too many white blood cells running wild and starting to cause harm instead of good.