What's wrong with the email based solution. It's dead simple to use for the user and most email clients offer some filerting to sort the mail from the mailinglist out to some folder, or the user can simply set up a separate email address for that. Furthermore most users have signed on to the odd mailinglist alerady so this new one behaves pretty much like the other ones.
What I absolutely do not like is the forum type solution where you have to go to some website which might be fine if you're looking at one or two only, but not at a dozen.
And with an NNTP interface there is the problem that that usually is more complicated to set up - for the message board which thus needs to run the NNTP server and for the user who has to configure the additional NNTP server etc. And opposed to the mail based solution what's the advantage (assuming you set the filter to put the mails for that list in a separate folder)?