"To start, we've improved the discussion flow by adding ownership and edit rights to individual comments. Each comment 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 document and view them later by clicking the discussions 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 notification. 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, discussions are only available for new documents. Google says that the explanation is that the new feature includes "a number of significant improvements".
{ Thanks, Andrew. }