Google Docs has always been a great collaboration tool, but the "comment" system left a bit to be desired. Comments were essentially sticky notes attached to text in the document, on which all parties could leave little messages. Google's completely revamped this feature, turning it into a more IM-like experience.
It has added contact pictures, making them easier to scan, a "resolve" button that lets you hide a discussion but review it later (instead of deleting it outright), and email notifications so you can see what your collaborators are doing while you're away. The feature will roll out over the next few days in new documents only, so keep an eye out for it, and check out the video above for a demonstration. [Google Docs Blog]