I am a fan of the extensibility of the CMSes. You can upload some code (usually PHP), authorize it from the CMS admin panel and it's running.

I wonder if it is possible in Google App Engine. I haven't checked the extensibility of existing CMSes for Google App Engine, but if there is any of them that supports plugins I would like to know how they did it, and whether they are JS plugins only, or if they support Python/Java plugins too.