AppJet was designed to be easy and make it possible to write cool little web apps with a small amount of code and a small amount of knowledge. From this perspective, it becomes obvious why we chose JavaScript as the server language: it reduces the number of languages you need to learn by a factor of 2.
I think it's succeeding at what it does. There are lots of cool apps in the app directory, and most are <500 lines of code in a single file. Many of these apps are written by high school students who learned programming with AppJet. Some of my favorite include {pxl,outliner,files,wikijet,agglodex,dict,livescore}.appjet.net.
We are working on improving the AppJet Framework to be good for large apps. We're also working a lot on our platform. (mrtron makes a good point about platform vs. framework elsewhere in this thread). One day, we'll tell the hacker news community that we think AppJet is a suitable replacement for PHP, Django, RoR, or whatever you're building your startup on. But we're not there yet.
In the meantime, if you want to hack up a simple database-backed web app without worrying about hosting or setting up SQL or anything, AppJet is your platform. Or if you want to learn programming with a platform that will let you do a lot knowing only a little, AppJet is your platform.