It appears that if someone is an Administrator (and likely a Moderator?) they can click on the little icon to the left of the word “Topic” on a category page to enter into a sort of bulk update mode where you can select multiple topics and make some change to a lot of them at once. Most of these changes are moderator abilities like deleting topics, moving, etc, however some of them are things that normal users can do (and in particular, setting the notification level).
It would be great to allow regular users to be able to access this menu in order to take a bunch of topics and change their notification level at once.