Good post. Sounds about right to me.

My hunch is that posting frequency is the killer benefit. And may be based on some non-linear effects in blogland : a couple more posts could push you over the psychological "frequency" watershed, so that people think of adding you to their aggregator and becoming regular readers.

As for the question of different voices, I think I expect different things from group-blogs than individual blogs. For example, with an individual, I respond well to an ideosyncratic mixture of interests. I want to know what comes out of the cross-fertilization between the themes. How Dave Winer mixes his baseball philosophy with software philosophy etc.

But with a group, I don't expect unrelated subject-matter to interact in the same way. So I prefer groups to be far more rigorously focused on a clear theme. I like Many2Many, but Crooked Timber, despite having excellent writers and many themes that appeal to me, still appears a bit too arbitrary, and I often forget to check it.

In a sense, I expect a group blog to be more like a traditional magazine : organized around a topic, and presenting a fairly comprehensive sampling of the news and discussion of that topic.
On the other hand, I don't necessarily expect the different writers to *agree*. If they argue and disagree about the topic, that can be particularly fascinating.