Action when a comment has been replied to
I've got something similar in setting up a forum, where all posts on a topic are really just comments. We've set it up to allow people to "subscribe" to the topic via the Flag module and then have a Rule set up to notify all subscribers after a new comment is added. Here's the Rule (from /admin/config/workflow/rules )



And here's how it looks when you click "Edit" in the screen above:



So basically, I've set up an event for a comment being added to a forum topic, then under actions, fetch all the users who have flagged that node and loop through, sending them an email (which can be customized with tokens based on the newly added comment). You can automatically flag a user to a node when they create it or comment on it.