Like one of the posters above mentioned, forums have multiple subsections. As an example let's say the forum is about animation, I can read Japanese animation section and ignore the Spanish animation section. With a mailing list it's all or nothing.
4chan is a forum, neogaf is a forum, gamedev.net is a forum. I think they all work much better as a forum than a mailing list.
I think mailing lists serve a different need/style/purpose than forums.
What dfc said, but other than that I think that is more an argument for usenet than for web forums? At least the "subscribe" step tends to be better integrated with newsreaders than with MUAs, so it's easier to subscribe to a whole bunch of groups. But on the other hand, subscription tends to be only a small part of the overall experience, so I am not so sure it really matters all that much.
How is that different than being subscribed to debian-spanish@l.d.o and not subscribing to debian-japanese@l.d.o? Or subscribing to debian-devel-spanish@l.d.o and and not subscribing to debian-devel-french or debian-devel-portugese?
For one it's easier to browse without committing. If there happens to be no new topics in Japanese Animation I might read Spanish Animation and if I want, reply to a topic. With N mailing lists I have to commit to the entire list rather than just individual topics in subforums. In other words I have to subscribe to the "Spanish Animation" list rather that just one topic inside.
4chan is a forum, neogaf is a forum, gamedev.net is a forum. I think they all work much better as a forum than a mailing list.
I think mailing lists serve a different need/style/purpose than forums.