There are two things I want to talk about. One is the underlying technology and the other is the user.
While there are tools which bridge the gap between the mailing list and the forum, I still haven't found them in common use. I'm going to set aside that line of discussion in this post and go after the everyday usage of the two tools.
- The Underlying Technology
A mailing list, or email in general, can be accessed in two ways.
A forum is only accessed online.
The two online access methods are quite different. IMAP access is usually quite fast since one email is downloaded at a time where most forum access requires frequent whole-page fetching and is significantly slower.
Most Webmail is still much faster than a forum because Webmail is completely focused on the email itself without the rest of the "experience" of a forum. A forum demands the whole "experience" of surfing the web. It is cluttered with irrelevancies like navigation, colour, avatars, signatures and such.
So right here we can see a speed and general attention problem with forums, which can be solved by current webmail technologies. Remove all the excess clutter and Ajax-ify the message reading experience. I'm sure someone somewhere out there is working on this right now.
- The User
There is a certain kind of user, and over time I'm more strongly becoming a member of that group, who have a sort of focus. When highly interested in a task, the rest of the world falls away for a time. They become agitated at the slightest distraction from their current task. Ergonomics becomes extremely important: The right keyboard, hotkeys, chair, screen settings, background sound level, temperature. A responsive computer becomes important. Very. Important.
The computer itself becomes an extension of the mind. Typing is so intuitive that there's no thought involved in it. Fingers fly across the keyboard at speeds unheard of in everyday people. If there were a faster way to input, it would be used and it would still be too slow. So when the computer "hiccups" for any reason at all, it's an amazing distraction to the train of thought.
It's that delicate train of thought which gives such a strong preference to email.
A highly-demanding user will use email and will download it locally. They will have all their native preferences including their favourite email client. Anything they can tweak to their preference will be. They can access emails swiftly with hotkeys. Replying becomes as fast as their computer. Control-R, typety-type-type, control-enter, n, n, delete, and so on. They can interact with the email like anything else on their computer. Controlling their environment and tools they can sharpen their attention span.
That same user becomes agitated using a forum. It's cluttered, it has none of their preferences, and none of the functionality they get from email. Searching requires a bunch of clicks, advertising wastes screen space, the layout is wrong, the colours are wrong. And it's damned slow. While a lot of that can be hacked, there's still no substitute to email. All the personal preference that's available through email is wrested away. Few things are controllable and nothing reinforces the developer mindset.
A web interface can be tidied up, but I cannot see how a web interface can ever replace a local program. I think few projects will be able to shift all their developers over to a forum, and that the rift between the forum and the mailing list is permanent.