As an example, Google App Engine uses Google Datastore, not a standard database, to store data. Does anybody have any tips for using Google Datastore instead of databases?
If you're used to thinking about ORM-mapped entities then that's basically how an entity-based datastore like Google's App Engine works. For something like joins, you can look at reference properties. You don't really need to be concerned about whether it uses BigTable for the backend or something else since the backend is abstracted by the GQL and Datastore API interfaces.