do take a look if you can leverage jsonb datastructure of postgres - you get 80% of the power of mongo and all the advantages of postgres.
https://www.compose.io/articles/is-postgresql-your-next-json...
https://www.compose.io/articles/is-postgresql-your-next-json...
But I would love it even more if it were easier to distribute (see replica sets in mongodb, with auto-failover and other goodies).
http://www.postgresql.org/docs/devel/static/functions-json.h...
That said, if you're doing a lot of mutation of large JSON documents stored in a single Postgres row value, the storage/concurrency control behavior still won't be ideal.