I'd like to second Terry's request for the ability to subscribe to threads, and email notification of PMs would be great too.
I can't remember where I read it (maybe codinghorror) but I recall an article reasoning why email subscriptions were the best way to kill a forum / community and I tend to agree.
I'm not opposed to a digest summary but the forum is meant to be a forum and not a mailing list. I think email subs encourage the post-and-run more than a true community discussion.
________________________________
Oh, I agree.
What I'm looking for is a mere notice:
"SIMON has posted on your Topic, "Werewolves in the Arctic."
A hyperlink directly to the topic speeds things considerably.