On meta.movies.SE we tend to have longer discussions in question comments about an issue. Sometimes comments will come in days or weeks later, and unless you are the person who posted the question, you don't have any chance to be informed about new contributions to the discussion. So my only option is to check on old questions regularly to see if anyone posted anything new in the comments section.
The @-syntax only helps if its a discussion between two persons (which most often it is not).
It would be great if it were possible to subscribe to comments on any question, not just your own.
This could either be done automatically when you "favorite" a question, or explicitly through a new button.
I guess this could be limited to the discussion tag on meta sites, but might be helpful in other cases as well.
To clarify: The reason I would need something like this is not for discussions in normal questions. This is about discussions in meta site discussion questions.