Google has just made it easy to collaborate with the introduction of discussions to Google Docs. The new functionality, which was introduced yesterday, allows users to take comments one step further, aggregating all of them into one location and attaching the commenter's image. Each comment also includes a time stamp so you can see exactly when it was posted; comments are displayed in chronological order.
You can also elect to receive email notifications if someone adds a comment to your discussion, and reply to comments directly from your email. Discussions are being rolled out to all Google Docs users with registered Google accounts now, and are just the latest in a series of upgrades to Google Docs that includes support for new file formats and more comprehensive mobile access.