I would like to suggest the change to the "From" field in email notifications: I think it would be very useful if user XXX makes comment in the task, and email is generated with "From" field being that user XXX (and his/her email address in reply).
If someone replies to the task-123@.... email, it goes back into the system as a comment and is broadcast to all people involved in the task instead of just to the user who made the comment.
I don't think just replying to the user making the comment would be much better.