Team Drive is based on a site that Fabien built years ago to record donations of items for a charity food drive. Users from an organization are split into separate teams, and then compete to see who can collect the most food items. By adding this friendly team competition to a normal food drive, you can increase the participation and results of the drive.
Building on GAE is quite different than traditional single server application deployments. Everything in App Engine forces you to think about how to scale your application. By placing constraints, such as a thirty second processing limit per request, it forces you to think about approaching problems differently than if you were writing this to run on a single server.
App Engine supports two types of authentication methods, Google Accounts and OpenID. If you want to leverage their session management you need to use one of these. For an site aimed at organizations, I didn't want users to have to create a google account or OpenID. To work around this I implemented an OpenID provider in the application. Administrators could upload a list of email addresses and a Team Drive OpenID would be created for each of these users. The users themselves don't need to know that they are using OpenID, as I only ask for their username and password.
Another example is for counting the number of items contributed by a person, team, organization or type. In a relational database you might just sum these up on demand. However using the Google datastore you generally store counts on the object itself and increment this count on each update. App Engine has also provided an implementation of map/reduce that makes it easy to perform operations such as sums or counts over large collections.