Well, something's working, because I keep getting e-mails telling me people have responded to threads I'm "subscribed" to -- that I posted on. I gotta turn that off or I'll spend all day reading e-mails!
I went through and set the defaults for all profiles so that "Notify?" is now set to No, but if you were already subscribed to a thread before that, you'll have to unsubscribe now to stop getting emails.
I've also fixed the unsubscribe / subscribe feature, so you can now use it through threads and the control panel.