An excerpt from a Google App Engine snapshot (see this page for the beginning of the article)
However, no matter how quick and easy building Web applications is with App Engine, and no matter how good Google's infrastructure is, the service's lack of openness remains a serious drawback. While Google's representatives say that they want to avoid locking companies into their system, the reality is that as long as important components such as the database remain proprietary, developers will have limited flexibility. In my case, I don't currently want to manage my blog so much as just write it: I just want software that works. Yet it was important to me to reserve the right to move it wherever I want, to add or remove tools, and possibly to learn enough at some point to begin participating in the design of the platform. In its current incarnation, App Engine doesn't give developers analogous options.