Google Dos updated its comments system to facilitate rapid and seamless discussions and integrate with email in an intuitive way. Since there are a number of significant improvements, this update is only available for newly created documents for now.
To start, Google Docs improved the discussion flow by adding ownership and edit rights to individual comments. Each comment now has a timestamp and profile picture.
Docs doesn't force you to delete comments. Instead, you can resolve comments to remove them from the visible document and view them later by clicking the discussions button at the top of any document. You can even re-open comments from here.
Docs integrated comments with your inbox to facilitate more rapid and responsive communication. You can now add someone to a discussion with an @mention. For e.g., typing @firstname.lastname@example.org in a discussion will send a notification email to Eddy with the text of your comment.
Eddy gets a message in her inbox with your question or comment. She can easily click through to open the document itself to respond, or simply reply to the email. The reply will show up in the document as soon as the email is sent.
To turn off email notifications for a document, just select Mute updates from this comment at the bottom of any email or select Discussion notification settings... from the Discussions drop down.