Obviously I can only speak for myself, but, in my opinion MongoDB is broken at a deep fundamental design level because the clever technological metaphor it's built upon (mmap is cache, we can overwrite in RAM and let the OS handle writes!) is just mistaken. All its other problems - trashing data, hogging resources, excessive locking, poor performance - are derived from that fundamental error, and all the fixes consist of beating a gradual retreat from it, a process rather like incrementally evolving a brick into a F1 race car. One has to ask, wouldn't it be better to start off without the brick? And rethinkdb is basically what MongoDB is, from a user's perspective, but it never had the brick.

It's a major indicator of poor system design.

EDIT: To be clear, I'm not necessarily knocking MongoDB—people who know more about it than I would do a better job of evaluating that. But realistically a swappable DB backend isn't that difficult, and if it is, they should seriously reconsider how they are using the database.

The back end is swappable. I think right now you can use Mongo or MySQL. If you want to use something else you have to write your own DDL. They hope that more people will do that in the future. Or presumably, they will do it themselves, once they think it's a priority.