Forgive me if this rambles a bit…
I don’t know if the goal is always to expand the audience. There does come a point where it is too big (at least for me). For instance, I rarely read any discussions on MacRumors because any subject that’s interesting ends up with several hundred comments - which consumes more time than I want to spend on a single subject.
One of the things I like about TidBITS is that I can read every comment posted to every thread. The quantity of comments never gets so large that I have to start ignoring threads due to a lack of time. Unlike MacRumors and others, where there simply isn’t enough time in a day to read everything, even if you want to.
Moderated discussions are important because flame wars, spam and other related things destroy the signal-to-noise ratio. I don’t want to wade through a site that’s 90% junk in order to read the 10% that’s interesting.
SlashDot attempts to solve this with their moderation and meta-moderation system. No comment gets deleted, but comments get moderated up and down on a scale of -1 through 5. Readers can select a threshold so they can choose to only see high-rated comments or they can choose to see everything, or any level in between.
This sort of works, but it can get weird if there are highly-rated comments in response to low-rated comments. You end up seeing only part of a discussion. Of course, you can choose to view the filtered comments. My biggest problem, however, is just that SlashDot is too popular. Just like MacRumors, it’s drinking from the firehose.
Stack Exchange works well, but it is not a discussion forum. It is a Q&A forum, highly optimized for people to ask questions and for others to answer the questions. The software and moderation system explicitly rejects discussion. This is why (for instance) anybody can edit anybody else’s answers, and even questions - the goal is to provide high quality responses to high quality questions, not discuss topics.
The author of the Stack Exchange software, Jeff Atwood, has a different software package designed for discussion. It’s called Discourse and is what TidBITS Talk is currently running.
iFixit’s forums are interesting, but they (at least the ones I’ve participated in) are focused entirely on Q&A related to their repair guides and teardowns. There’s a thread of comments associated with each step of the guide in addition to one for the guide itself. But that’s clearly not appropriate for any other kind of forum.
I personally don’t see a problem having a different set of rules for TidBITS articles and TidBITS talk threads. As @ace wrote, the former should be questions and contributions to the article, while the latter should be discussion related to the message that started the thread.
One feature I’ve seen that I’m on the fence over is automatically locking threads after a while. For instance, the comments associated with articles on opensource.com are automatically locked some time after the article goes live (I think it’s a month or two). On the one hand, it prevents people from discussing topics that may no longer be relevant, but on the other hand, it means you can’t contribute if you read an old article and want to make a point that nobody else made.