The Power To Un-Follow
In Twitter, as in other on-line social media/networking communities, you can choose whose messages you see and whose you don't. There are also features to hide what you say from anyone you don't specifically befriend and to completely prevent someone from sending you anything, even a form request to befriend you. These features give users complete control over who they listen to, and who can listen to, or even contact them. It's completely opt-in. No one has any "right" to throw anything at you that you don't want.
This has led to a quite enjoyable environment, and an evolution of groups of people who mutually befriend each other, and share the same sense of proper behavior. There aren't any "haters" on Twitter that last longer than a couple of days because everyone else simply stops following them.
Now, you might argue that this leads to groups who only agree with each other and shut out what they don't want to hear. To an extent this is true. Most of the people in my group of roughly 220 followers/followees I think tend to be politically liberal and tend not to discuss religion or politics. But it is not so much what you say on Twitter as how you say it. We were just commenting last night that we do have a mix of political and religious sentiment across the group, but people tend to not rant, accuse or polarize discussions. That isn't to say it doesn't happen from time to time. I'm probably one of the worst offenders, to tell the truth, but we all endeavor to keep our emotions in check and cut each other considerable slack, realizing that every once in a while something will touch a nerve and we just have to blow off a little steam. I can tell I've done that when almost no one replies to what I've said. The silence is a pretty good indication that you've gone a bit overboard and need to watch what you're saying.
The worst fate on Twitter is to not be followed. It's the online equivalent of the Old World practice of shunning. I've lost a follower or two in my time and it's not a pleasant experience.
And companies that try to join Twitter, befriend a zillion people and then spam them with output-only announcements never make it out of the batter's box. Almost nobody follows them. So, this has the effect of companies not even trying to spam, because it's just not worth the effort.
So what I've discovered from Twitter's "rules of engagement" is that giving people absolute control of what they hear and see is good thing. It's not a sin to be able to shut out people who act badly. It makes people a little more considerate and well-behaved. I think it's the answer to blog comment spam, too.