If you look for rules and regulations of Google App Engine you might feel you are being restricted too much. Apps only work as callbacks and must respond within 30 seconds, no file system write access, cannot create a thread and so on.
Datastore plus Bigtable gives great scalability to Google App Engine. Unlike the relational database technology datastore can scale horizontally. Bigtable allows infinite number of rows and columns and new columns can be added on the fly. It scales out by adding more servers to the datastore cluster. GQL has SQL like syntax and intentionally does not support query joins. So overall there is not much of learning curve involved. This is very much the case with startups where they can neither afford to have buffer nor to devote time for learning phase.
You focus on development and let Google take care of IaaS complexities! That is so conducive for startups. If you look at the goodies provided by GAE- Memcache, URLFetch, Task Queues, Cron Jobs and the list is apparently endless. So what are you thinking? Go grab the advantages of GAE!