I would like to suggest a notification indicator in the grade book for new submission comments. E-mail notifications are already an option but when you are physically in the grade book there is no way of knowing that a new comment has been made. When conversations are started in the grade book a new comment can be easily missed.
Mock up screen shot of what it could look like:
![A notification indicator icon displaying when a new submission comment is left and is unread by the user A notification indicator icon displaying when a new submission comment is left and is unread by the user](/t5/image/serverpage/image-id/45082iE2D50BAB695BEFC1/image-size/large?v=v2&px=999)