When dealing with forums, then keeping user notified of new posts is crucial component of success.

The In-Bulletin sends e-mail about new posts in following cases:

Activity Happened in Forums	E-mail Recipients
post is modified	administrator
topic is added	administrator
post is added to existing topic	topic owner, but only when he has subscribed to topic updates (off by default) user, who has created that post

There several drawbacks in current notification system:

user won't receive any notifications about his own topics until he manually enables such setting:
user, who added new post is getting e-mail about that, but this isn't very useful unless for spamming purposes
users who participated in discussion (added posts to the topic in past) aren't notified about changes made to the topic

Solution

implicitly enable "Notify me about replies" and "Notify me about Private Messages" settings (will be enabled unless user have manually disabled them)
notify all participants in the discussion (except user have added a post)
enable notifications for all existing topics in upgrade script
subscribe (via kSubscriptionManager class) users to every topic where they have added new posts (only if they aren't these topic owners) in upgrade script

In-Portal Forums

Once ready I strongly recommend applying described above changes to the In-Portal Forums as well.

Here are some statistics from In-Portal Forums: