Triage the ones you really don't care about by entering the topic and selecting "normal" from the notification state in the right gutter (or on mobile, expand the progress bar to see what would normally be in the right gutter.)
( note that this does not require you to be at the bottom of the topic, cc @clay )
Then when you are done, and have only the "good" or "bad" topics left in unread, use the dismiss button. You are given a choice when pressing the button of just resetting unread count or "to hell with this topic forever". Try it.