Mailing lists are searchable, but harder than a forum to search. Forums let you search by any of text, title, author, date, category,
Ok, that sounds nice.
The 'category' thing sounds tricky to implement on top of a mailing list (as I would like it)...
Yes, categories are nice.
You can't cancel or edit posts. Forum moderators can delete spam posts.
That's just a horrible feature. As with any kind of censorship it will do nothing but make people suspect you for deleting legitimate posts.
There've been forums like that, but they die quickly. Only forums with trustworthy admins get popular. So that's generally not a problem.
Spam should be removed (or maybe categorized under 'spam') by a spam filter. And if one does slip through, it will disappear in the noise in no time. Aren't most forums like about 30.000 pages long? Would you ever notice anything like a single spam mail?
Yes, most people do notice them, because even popular fast moving forums aren't noisy - they lists thread titles instead of posts. It's a lot more concise that way - you don't get bured in posts. For some reason spammers never post in existing threads, they always create new ones. And at every active forum I've seen, their posts always get noticed, reported, and moved or removed.