I agree that there's a whole lot of Mongo hate on here, but it falls into two categories:
1) Teams who started with Mongo (which makes sense when your schema is still in flux and you are still feeling out what you want it to finally be) then stuck with it when they should have moved to an RDBMS due to their use case, but never did and then find out that Mongo doesn't support features that any RDBMS from the 90's would.
2) Old school RDBMS people who are, as Steve Yegge would say "software conservatives." They love the way RDBMS's force people to really put thought into their data modeling upfront, and believe that anything that allows people more flexibility is opening the door to increasing amounts of chaos. Plus they hate the thought of returning to the dark ages when databases didn't all use SQL, and everyone had to learn a new query language for a new DB.
FYI: I've heard Meteor is planning on allowing use of other databases. I would particularly be keen on use of RethinkDB. I really like what those guys have been doing so far.
That doesn't seem very accurate. Plenty of "how do I shot database?" people are very anti-mongo too. Some of that may well be because they feel mongo gets more mindshare than their preferred nondb, but don't blame postgresql users for that.
