Looking through these forums, you'd think that nearly every DF dungeon run ended with people being kicked, yelled at, or otherwise harassed.

In the 2 years and hundreds of dungeons I've run, I've seen a grand total of two runs that went south due to people being rude, belligerent, or just flat-out incompetent. I've been kicked from a run a grand total of once, and that one was very random (it seemed to be going fine, we had just cleared the second boss, no one had said a word, and I suddenly found myself back in Idyllshire. Odd.).

Granted, I'm a filthy casual who doesn't really do raiding much, so maybe that community is different. But the overwhelming majority of people I've seen have been positive, friendly, and helpful. YMMV.