This has been discussed at least once before but it was accepted and implemented but not the way it was originally intended. If I overlooked an already existing identical feature request, sorry.
When commenting on questions from new users, mostly asking for clarification or additional information, I often find that these users have replied, but forgot, or didn't know, to address me using
@username. I will find some of those replies but only by accident, when browsing through questions. Askers will be waiting for feedback and never receive any. (At least not from me.)
I think this is a real problem.
Here is a list of real life examples. Obviously, this can only be the tip of the iceberg as those are the question I happen upon by accident later.
I propose that either
notifications about subsequent comments be sent to every person who has commented in a thread. ("Facebook style notification"). The vast majority of comment threads on SO is max. 2-3 comments long, I don't think it will clutter people's inboxes.
or: Facebook style notification be activated for questions only so abovementioned situation doesn't happen
or: Newbies are shown a big fat info that if you want to reply to a previous comment, please include the user's name prefixed with a
@(I have not checked with a low-rep account whether this may already be the case, but from the number of "lost" replies I assume it isn't.)