Recently I commented on Gershon Gold's answer here. Shortly thereafter (about 5 minutes), I noticed that friendly red notification icon in the top left corner. It was notifying me that Gershon had responded to my comment.
Just one thing. He never @mentioned my name. Nor is there the little pen icon next to his post, so it can't be that he mentioned me and then edited it out.
Jeff Atwood has said:
2. Response must include @username that you are referring to, where "username" is a reasonable match to the user's current display name (as seen in the comments above yours).
But is there another way to get notified by comments, besides for @mentions?