You can't just port your application to App Engine. You have to start from scratch. Google enforces all the right constrains to force you to write scalable apps

You can't just port your application to App Engine. You have to start from scratch. Google enforces all the right constrains to force you to write scalable apps. The killer feature is that there is no need for system administration or maintenance. No need to setup dns rules to make your email go to the recipients, no need to update apache or linux, no need to setup an xmpp serrver, etc. Plus, you can leverage Google's battle tested infrastructure. The icing of the cake is that you can now use the internet programming language, ie JavaScript, while still having access to the vast collection of Java libraries. A match made in heaven if you ask me!