Maintaining connections in the face of failure is a relatively little concern, what I perceive as the big problem here is having the database handle an arbitrary application with zero application-specific administration.

Google's datastore promises that if you write your app for their platform and it works on the small scale, it will scale out without problems. Datastore latency is constant regardless of how many records you have there. How will you scale a join between two tables, each being partitioned between multiple different servers, transparently for the app?