> Cassandra is one of the better ones out there

I take it you haven't actually used Cassandra much in the last few years. It's data model is almost identical to a typical relational one and it's consistency promises are quite clear:

http://www.datastax.com/documentation/cql/3.0/cql/aboutCQL.h...

And I've scaled Cassandra clusters from 1 to 100 nodes in hours with no issues. It really is quite simple. Likewise have had no issues with MongoDB replica sets. It is definitely not "really, really hard".

> postgres is to do it right, or not do it

What a pathetic cop out. PostgreSQL has been around for decades they've had plenty of time to have a proven, stable solution implemented.