Today, we have some more improvements coming in XenForo 1.4. Individually, these features are not big enough to dedicate an entire HYS thread, but some of them have been heavily requested nonetheless.
Limiting watched content notifications to active users
As you can probably work out from the screenshot, if this option is enabled, users that have not visited recently will not receive (or be considered for) alerts and emails from content they watch. This is mostly done for performance reasons. However, you can probably assume that if a user doesn't visit your forum for a long period of time, they aren't interested in the content they're watching and are unlikely to return. Stopping emails to them may prevent an invalid spam report...