"A developer might not know how to optimize a slow-running Postgre query, but he knows how to cache data and denormalize in a manner that makes the query he wan

"A developer might not know how to optimize a slow-running Postgre query, but he knows how to cache data and denormalize in a manner that makes the query he wants to run fast" And you can cache and denormalize with postgres just as easily, and it will probably perform better and not corrupt your data. What mongo makes easy is things like replication and failover. It can go horribly wrong on that front, but until you see that, it is much easier to get up and running with replication and failover.