Here are some common ones. Open newsgroups frequently erupt into flame wars over earth-shatteringly important topics like spelling. Putting a list of frequently-asked questions online is one way to cut down on this, but it'll never go away. People will always do this - posting a question about bicycles to the cooking SIG or whatever. And then insist that the question is, in fact, highly relevant to the topic at hand.
People often post incredibly long rambling posts that go nowhere.
Inappropriate subject lines. People will always post messages with subject headings that bear no relation to the content of their message, which can be a real time-waster. With an open newsgroup the SIG Organizer's job is to guide discussion, politely make suggestions when things are going way off topic and introducing new ideas when discussion dies down. The SIG Organizer will also serve as a patient online teacher as well, helping newcomers get up to speed in a polite and respectful way is an important part of moderating a group. SIG Organizers should also help to reduce flamewars and other high-tension moments.