I actually like reading the articles about police misconduct, police actions in the news, or just about arrests and raids. The discussion can be very good and very useful. I admit it can get out of control quick though.
The problems I see are:
1. Lack of reading comprehension. Some posters miss facts of the case or seemingly make them up. A few posters get a certain scenario in their heads and argue based on that fiction. It throws wrenches into otherwise simple disucussions.
2. Ranting. Some posters just look for issues to get charged up about and rant. Rants not always being related to the case in question.
3. Baiting. Some posters don't rant or such very much, but they love to throw out rant bait to get under the skin of others. Usually they sound reasonable on the surface, but it can be pretty obvious that they are not interested in a serious discussion just seeing how much havoc they can cause. I bet they are not normally the ones getting banned.
4. Arguing over unrelated points. Related to #1. I have seen multipage arguments where each side basically agreed, but was saying it differently enough they were arguing about nothing.
5. Poor written communication skills. I really think a lot of people think what they said is not bad, but say it so poorly that it is taken as something completely different. I have caught myself doing this before. Had to reword half the post. Also, there are a lot of people who should be forced to take persuasive writing classes.
6. Being able to separate the argument from the poster. One thing I try to make sure I do is avoid attacking the poster, but rather attacking the argument. I'm sure I have not done it always, but I have edited posts on this and other sites quite often to take out stuff I regret saying as soon as I posted it. I have also written long posts that I have just canceled/deleted feeling it was better to say nothing.
I guess there could be more or less depending on how detailed you get. I am sure I have been guilty of one or the other in the past.
One other thing: I don't want anyone who got banned to take it personally, but I have seen mods warn people before about what was obviously a bad post or series of posts; then the poster argues about it like they were as innocent as a babe. It is the same reason I don't take one-sided news articles at face value. Some people just refuse to admit they were wrong.