From time to time it happens that I'm not notified about comments addressed to me. This is because correspondents forget to insert
@username. I know there can be another reasons, but let me leave them out the consideration, because the most times when I do not receive a notification, it's due to the abovementioned omission. I think other people do also encounter the same problem. This makes it difficult to provide prompt feedback and collaborate in timely manner. Below is what I think could diminish the problem. First method keeps existing fine-grained approach. Second method makes notifications more coarse and breaks back-compatibility with current approach, so it's more questionable.
SO could check if a comment being posted does not contain a username tag and suggest to choose one from a list of current participants. This should happen only if SO can not deduce a person to be notified automatically (as it does now for example if there is only one commenter). This is almost done at the moment, but the list is shown only if a commenter enters
@symbol. I propose to show the list before submission and without any action of the commenter.
SO could send notifications about any changes in comments for questions and answers where I participated, and if
@usernametag does not explicitly point to someone else. The reasoning for this is that a person that has already posted something to a page is likely interested in its subject and could find further discussion relevant as well. This mimics chat metaphor, where any message without username is considered public.