[GI_Josh]

That's a good point, that is clearer to what the article is trying to say.

Defining a community by its ability to exclude is interesting. Twitter just puts the impetus to exclude on a personal level (person A blocks person B) rather than on a community level (person B is banned from this IRC channel)

[schoen]

Yes, I think that's right. And some unmoderated mailing lists and newsgroups have also favored that approach (with killfiles), but even there there is potentially a stronger threshold for joining (you have to deliberately subscribe to the list or group) and stronger recourse for extreme misbehavior (at least on mailing lists, where someone can be banned from the list).