There's this thing called "filters". Mailing list messages don't need to touch your INBOX, they can be easily and automatically filed in a folder. I have one for each mailing list I'm subscribed to, my mail client does better threading than a forum, better searching, and I don't have to go hunting for new messages, they come straight to me, and my mailer neatly tells me which ones are unread (and with a simple keypress I can go to the next unread). When I'm a newcomer or not a subscriber, I can search in the archives (I prefer gmane, but in a pinch google will do).

Well, I agree on this one. One of three, not bad
A forum sucks: you have to go hunting for new messages (and meanwhile you're also downloading messages that you already read), there's usually no good threading support and it's more difficult to keep track of everything. Oh, and I don't have a local copy of the forum (my mail store is always there in case of need, though from time to time I just purge old messages).
It's difficult to follow more than a handful of forums, while it's much more efficient to be subscribed to various dozens mailing lists.
Sure, you usually (but not in all cases) have to subscribe, but I find it less intrusive than registering at yet-another-forum.