“Open source web application development doesn’t seem sustainable. Deployment concerns are a huge part of this. Being interested in a project requires that you be able to use the product (and to use it casually). Right now most people can’t use open source web applications.

There’s some applications, sure. WordPress, Trac, MediaWiki, MoinMoin. But most wiki software doesn’t have a vibrant community. Many a bug tracker has fallen by the wayside. Blog software projects have a horrible time building a viable community. Other website software hardly gets anywhere at all. A lot of the development that might appear to be application development really is more like a framework when you look closely (e.g., Plone, Drupal). Given its better deployment story, it’s no surprise PHP is the basis of most viable open source web applications.