these blogs succeed ... because they ... exclude comments whose quality falls below a certain threshold.
I see an opportunity for philanthropy. Identity the elite people that one hopes will blog, and then pay for somebody else to do the comment moderation for them.
The problem I foresee is that this turns out to be big-money philanthropy. Who do you hire as your moderator? They probably need a PhD in mathematics, and the right personality: agreeable yet firm. People like that have lots of well paid options in which they are not playing second fiddle. The philanthropist backing this may have to come up with $150,000 a year to pay the wage bill.
Turning this round, it answers the question of why so few elites blog. Hoping that they take on the task of doing their own comment moderation and community building is hoping that they engage in some serious philanthropy and tolerate getting little credit (because they are paying in kind rather than in big wodges of cash).
The problem I foresee is that this turns out to be big-money philanthropy. Who do you hire as your moderator?
Money isn't the only thing that motivates people. Most people who moderate forums on the internet aren't payed to do so.
Something that's caught my attention over the past few months is that out of the strongest thinkers who I'm familiar with, a very low proportion record their informal ideas online, and discuss their thoughts in the public domain.
The domain that I'm most familiar with is pure math, so I'll focus on that, but I imagine that my remarks apply more broadly.
A very large fraction of pure mathematicians (including a large fraction of elite mathematicians) post their papers to ArXiv, a database which stores preprints of mathematical papers. The invention and widespread use of ArXiv has been very valuable, in that researchers can easily notice and access the new papers in their fields as soon as they become available.
That not withstanding, the fraction of mathematical thinking that's in the public domain is vastly smaller. Math papers generally don't include explanations of why the researchers are interested in the questions that they're writing about or how they came up with the proofs of the theorems. Even when an author does attempt to explain his or her thinking, a reader will often find parts of it opaque, and want clarification. The need to write to the author for clarification poses a trivial inconvenience which, in practice, discourages a large fraction of questions that people ask.
Informal mathematical thoughts are extremely important for doing mathematical research, and it's generally difficult to learn it without in-person contact with authors of papers.
A natural solution to these issues is for researchers to spend more time blogging about their informal thoughts, and for a commenting system to be enabled for readers to offer suggestions or request clarification.
The mathematicians who have the most to offer in the way of ideas and insight are the elite mathematicians. The fraction of them who blog is tiny. Terence Tao and Timothy Gowers do a considerable amount of blogging, but they're nearly alone in this.