Currently, automatic forum subscriptions are handled at a group level. Anyone who is a part of that group will be emailed when new topics are posted in a forum. However, the user has no control over it. It's an all-or-nothing approach that is controlled by the administrator, not the user.
The current aproach is okay for certain situations and it has it's place, but it certainly isn't ideal for all cases.
I'm running a user support and disucssion forum. I'd like to subscribe them to all forums initially, and then give them the control to unsubscribe from any forum that they don't want to partake in.
Thus, I'd like to see a different approach. The algorithm would go something like this: when a new user is created (or when a member's group is changed) then subscribe them to all the forums that they are allowed to access. And by "subscribe them", I mean set it in their personal subscriptions. That way, they can go into their own My Controls->Subscriptions->View Forums area and unsubscribe or change digest options on the forums that they were initially subscribed to.
You could have a setting to toggle this feature on and off in the ACP. Or just use the group-based subscriptions. For my needs though, I'd turn off the group-based subscriptions and have some rules set up to do a 1-time subscription for all users who are put in a specified group.