It is a fully-managed application platform. So far, I do not know a platform which comes close to GAE's full package: log management, mail delivery, scaling, memcache, image manipulation, distributed Cron jobs, load balancing, version management, task queue, search, performance analysis, cloud debugging, content delivery network - and that is not even mentioning auxiliary services that have popped up on Google's cloud in the meantime like SQL, BigQuery, file storage... the list goes on.
I think the promise was never actually realized. When first released, you didn't have to worry about load balancing instances or scaling, it was "internet scale", out of the box, dependent only on your traffic. Then they went to the per instance pricing and raised prices.
I was really disappointed no service ever arose to fill this gap; internet scale as service you could say.