With AppJet, you write your entire app using JavaScript, including the server logic and database. This simplifies the process of building a web app, because it lets you do everything in just one language. JavaScript is easy to learn, but still pulls its weight for advanced uses. In fact, the AppJet site itself and the AppJet framework are written in server-side JavaScript.
The first thing I thought of when I saw this is a web hackers wet dream. A host and framework for creating javascript apps with easy hooks for storage and HTTP request/response objects.