One of the reasons I really like Google Groups is that it lets me treat something as a mailing list. By and large, I want to use an email client for serious discussions, not a 'forum' thing. Adding this functionality would be super cool.
We're launching email notifications next week.
Back when I first got online and only had one forum I was interested in, this was true - but it really doesn't scale. Trying to keep up with even 3 active forums by visiting each in turn is painful. Since then, I've pretty much ignored any information source that doesn't have either RSS or email feeds, since I can multiplex hundreds of those with no problem :P
(For a more average-joe version of this, note how facebook has largely replaced independent forums and IRC - people are lazy, and going out of their way to check an external site on a daily basis is much more work than clicking a "like" button and having that site's news become part of their standard routine)