This is key and often overlooked - MongoDB is so popular not because it's the best database but because it's so easy to get started with. Download/unzip/run to have a database engine ready. It also helps that you can also immediately store anything without any prior setup steps.
Postgres/mysql/sqlserver/etc are nowhere near as easy to install, as fast to get started with or as portable to move around.
Postgres members should listen this and have a simple getting started guide for osx, Windows, Linux. I tried brew install postgresql. There was no single place which tells me how to start server, access command line, create db etc.
On OSX there is the fantastic http://postgresapp.com/ . It installs into /Applications so it is easy to remove, and comes with a start/stop GUI and taskbar icon. Great for local development.
But installing and configuring Postgres "properly" on a server is still something of a challenge. Do I need to modify random_page_cost on a SSD or not? What are good memory limits on modern big servers? What exactly needs to go into pg_hba.conf?
None of these seem too difficult after reading a few tutorials and wikis, but it would be nice if the server set itself up with reasonable defaults based on the machine its running on.
Getting started with PostgreSQL on Linux is actually trivial. What is annoying though that there are lots of guides which talk about editing pg_hba.conf which is not necessary for the simplest setup. The default pg_hba.conf is good in most distros.
We must have different definitions of trivial compared to what I had to go through every time - it's a mess of an install process that takes tweaking config files just to have it even listen to external requests.
With the ease of services like AWS, we never installed a database server. Pick a database flavor, version, click, click and you're up. I suppose designing the schema take a little effort, but I find it much easier than properly architecting software.
Many if not most installations are still being done on actual dev machines and servers. While RDS and other managed services are nice, they're just a small fraction of the usage.
Also the fact that managed services help so much only speaks to the fact of how difficult these relational databases typically are to work with operationally.
if you're on a mac you can download postgresql.app[0] which produces a small icon in the top right status bar. You don't have to install users or permissions or anything it's super easy to set up. Getting it on prod can come later but for the first five minutes it works.
(granted, this neglects contrib extensions like hstore)
It's not just installing for development, mongodb (other than its ridiculous clustering) is very easy to install on production servers as well and moving an installation is basically just zipping up the folder and moving it somewhere else.
That sets up postgres. It doesn't let you get started doing CRUD operations inside your postgres though - which is where MongoDB shines, "just store my data, fuck it"
Postgres/mysql/sqlserver/etc are nowhere near as easy to install, as fast to get started with or as portable to move around.