One of the most interesting things about the GAE to me is the non-relational data store. I've blogged only recently about my doubts about that approach.
A friend of mine from the big G go back to me about this. "To answer your question about what's wrong with a real database, the datastore is all about scalability. The API trades off flexibility with access paths and transaction boundaries for the ability to store terrifyingly huge volumes of data, while serving a bazillion concurrent accesses.
Compared to an RDBMS, I find this style of data storage requires a bit more thought and design up front. However, almost every RDBMS based app I've ever written has required me to go back and optimise my schema and queries any way, so overall, schema-related work is about the same. Not having SQL does make running arbitrary reports and so forth more difficult."
Hmmmm. Now, coming from Google, this bears thinking about. Scalability as an issue I can see. We do have to put in a lot of work to keep guardian.co.uk performant, and it's almost all at the database, fiddling with schema and queries level. Much to think about...
Other people's thoughts on this; Google Datastore and the shift from a RDBMS and Google AppEngine, BigTable and why RDBMS mentality is harmful.
More on the GAE: