Basically, Google's playground is a way for developers to walk through OAuth interactions from start to finish.
Pick the APIs that you want to access (everything from Google Analytics to YouTube), and then start by going through the full flow. You can even set up custom endpoints to test out non-Google APIs as long as they support OAuth 2.0 draft 10. The Playground will show you the full HTTP requests and responses for each step. If you need to come back later you can generate a link that will reset the interaction to the current state, no doubt very helpful in debugging.
Google also has a forum for people who want to discuss the playground, specifically. What do you think? Is this something you'll be using?
You might also want to read Justin Smith's piece on changes to Oauth 2.0 endpoints that are due on November 15th. You'll see changes to the error responses and more. See the OAuth2-dev Google group for questions on those issues.