Now I don't know if this holds true for all newsgroups, but as for these two, the brevity rule is well in effect. In class, we also discussed the art of flaming. As stated earlier, flaming is simply an aggressive retort to a person's posted message. Although I haven't witnessed any such acts in my newsgroups, I do have an idea of what sort of messages would elicit flaming. It is these types of postings which go against the beliefs of the newsgroup that I think would elicit flaming.
People start new threads all the time.
Well, my feeling is because either they don't want to talk about a certain topic anymore, or they just simply have a new topic to discuss. Rather than keep posting follow-ups on the same thread, especially if there is something else which the person wants to say, a new thread can be started. If they do have something relevant to say on the topic, however, then a follow-up would be more appropriate. Threads keep going because of interest in that particular topic. It all depends on how much interest the thread provokes.