All apps need to be able to store information on behalf of the user. We've had a couple of different approaches to work with that don't require an identity system -- cookies and local storage. These are good but the data is bound to a machine, not to a person. So if you went to a different machine, you'd have to start over.
So what then? If you use a social network's identity system, you only get to store stuff in their data structures. For example, with Facebook, which has a very simple and elegant toolkit, you can create all kinds of messages. But what if you want to store some prefs on behalf of the user? Or the source code for the content? You have to start over from zero.
Three core technologies: Node.js for the runtime, Twitter for identity, and S3 for storage.
PS: I cross-posted this on Facebook. In the comments I wrote what's sure to be the next post in this series.