Document comments are a great idea. However, they create a 3rd (inbox, notification stream, document view) or even 4th place (notepad, inbox, notification stream, document view) place to check for to do's. This starts to break the usability for me. Actually I check a 5th place, my tray and a 6th place a personal doc with long term ideas.
I would thus suggest 3 things:
  1. Allow reminders to be associated with a piece of text in a doc, the same way comments are associated with it. Today's "comment" button would become a "Associated reminder" button.
  2. These comments/reminders should show in the inbox / delegated reminders tab. Maybe also offer more sorting /filtering options in the inbox
  3. Send a notification to the notification stream for reminders (which people can turn on/off in the notification settings).