Google Docs enhanced the commenting system, by addition support for conversations.
"To start, we've enhanced the discussion flow by adding ownership and edit rights to personality comments. Each observation now has a timestamp and profile picture. Google Docs doesn't force you to delete comments. Instead, you can resolve comments to remove them from the visible text and view them later by clicking the deliberations button at the top of any document," mentions Google.
One of the most useful new features is the support for notifications, which works just like in Google Buzz. Reply to a comment and the author of that comment will get an email announcement. He'll be able to answer to your reply from the email interface, without having to visit Google Docs. Google also sends notifications if you're mentioned in a thread.
Unfortunately, deliberations are only available for new documents. Google says that the explanation is that the new feature includes "a number of important improvements".