When Google released the new version of Google Docs, one of the missing features was comparing revisions. The latest update to Google Docs brings this feature back, but only for consecutive revisions.
Google Docs adds a sidebar that lists the most recent revisions and lets you preview them. "Click on a time stamp in the right column to see what changes were made at a given time or use the arrow keys to quickly scan through many revisions. Changes are color-coded based on each collaborator, making it easy to tell what has been added or deleted," explains Google.
Google groups the changes from a short period of time into a single revision, but you can click on "Show more detailed revisions" to see all the changes.
You can also use the up/down keys to scan through revisions and press Escape to go back to the editor.