I suggest to extend the subscrition model to include whole categories and / or tags.
These days users are used to get actively involved, and they hardly browse the forum by themselves looking for new topics of interest.
It could be implemented by substituting the "Subscribe" flag, in to the reply form, with:
- 'Subscribe this topic'
- 'Subscribe all topics in this category'
- 'Subscribe all topics with the same tags'
The two new subscribing features would only send a notification when a new topic is started (not replies).
The opt-out (unscribe) could be included in the notification itself.