While I believe MySQL is as well, it's hard to see progress/what exactly they are working on.
Pretty much the main reason is Monty is working on MariaDB, not MySQL. And he's pretty amazing.
Not something I'd like to risk my own company's database on.
PostgreSQL is a completely different database system, and is definitely preferable if starting from scratch, but migrating from MySQL to it can be a pain.
Being funded by VCs is not a bad thing in and of itself, but since most VC funded companies seek an exit strategy, it makes the stewardship of MariaDB at risk of being purchased by the next Sun or Oracle. In an ideal world (ideal for the future of MariaDB, that is), SkySQL would want to be acquired by Google or Facebook, but those two companies have thrown their lot in with MySQL by way of WebScaleSQL.
Disclaimer, I work for Percona.
Here's a nice summary page regarding features: https://mariadb.com/kb/en/mariadb/mariadb-vs-mysql-features/
Being more open sourced means there's a lot less friction when trying to deploy and maintain your MySQL instance(s).
It's one of the big advantages of FOSS: no vendor lock-in.
A software being open source means that you can deploy for any task in any environment, at any number, without the need to ask anybody's permission.
It being proprietary means that you must ask permission first, and will probably need to keep each piece of software under control, under different terms.
* Slightly better performance IIRC
* Not controlled by Oracle, who will likely kill off MySQL to promote their own database at some point in the not-too-distant future.
To be fair, if they were going to do this, they wouldn't have invested the thousands of hours required to release 5.6, 5.7, Fabric, the mysql toolkit, etc. I've talked to the developers in person who work on MySQL, and they talk quite a bit about the future of MySQL.
They have also acquired a number of MySQL based companies, and are committed to supporting them on MySQL.
A better strategy is the one they currently use - selling support tools (like Enterprise Support, Enterprise Monitor, and Enterprise Backup) to support the development. It never ceases to amaze me how much people will pay for these tools and services to support a OSS product.
I'm guessing there is a sizable slice of applications that would find PostgreSQL to be "good enough", and not need to buy Oracle subsequently.
Perhaps that's just paranoid, and Oracle is running MySQL out of the goodness of its corporate heart?
Based on personal experience, when databases hit that point, the logical next step is PostgreSQL if you're going to go through the pain of changing database engines. Or just do what $oldjob did and throw more and more resources at MySQL (256GB memory, anyone?).
It's the Oracle for orgs that can't afford Oracle.