Once upon a time, the thing for open source projects to have was one or more mailing lists (a '-user' and a '-devel' split was common). These days, an increasingly common option is some form of forum (or sometimes a chat system), to the point where people will sometimes turn up on mailing lists and ask if there's a forum version. As an old timer I often rolled my eyes at this shift, but it recently occurred to me that there is something to be said for forums in practice. And that is that in practice forums are much better at having separate threads of discussion and even entire topic areas.

In theory, mail clients can group messages by thread, let people mute entire threads that aren't of interest to them, and layer on additional things for topic areas and so on. In practice this relies on mail clients both being pretty sophisticated and doing the right thing on replies, which means that in practice it can be fragile. The default experience of an active mailing list with a mail client is a steady rain of relatively undifferentiated email. By contrast, forums don't give you any choice about the matter; your message or reply will be associated with a specific, distinct thread (and possibly in a specific topic area). This is more or less enforced by both the software and the social expectations; even if you can technically do otherwise with enough work, it won't get you the results you want.

In turn this matters because threads and topic areas are major filtering mechanisms. If you ask a question on a thread it's easy to watch the thread to find replies, and if you're not interested in a thread or an entire topic, it's easy to not look at it at all. In a sense, forums are optimized for skimming. Mailing lists aren't; mailing lists are by default a firehose and it's up to you to figure out how to skim them.

(Another way to put this is that in a forum, the default is to not read things. In a mailing list, the default is to read things.)

I've come to believe that this has the follow on effect that forums are easier for newcomers to ask questions in and for experienced people to stick around in. Newcomers don't have to swallow the firehose; they can start a thread with their question and watch only the thread, or cherry pick threads to read that might be applicable. Experienced people can selectively expose themselves to however much volume they want by looking at only some topics, threads, and so on.

In a low volume environment everyone probably benefits from the mailing list approach; the experienced people are probably interested in everything and the newcomers won't be overwhelmed by other content as their question gets answered, and in the meantime all of the experienced people will likely see it. But as the volume goes up I think filtering becomes more important for both groups, and especially I've come to think it's critical for keeping experienced people around.

(All of this makes me rather more sympathetic with projects that have chosen forums instead of mailing lists than I used to be.)