Even if I'm amazed by the performance of Postgres for this particular task (considering also that is a relatively new feature), I don't think performance is the reason why people are using NoSQL dbs. The problem that NoSQL dbs are helping with is scaling. I don't see this as a priority for an RDBMS such as Postgres. Take for instance MongoDB (just because it was named by many of the other comments here, but I guess the same apply to Couch or others): it's relatively simple to deploy a cluster with automatic sharding, replica, failover, etc.. because these are all builtin features.