* comprehensive transaction model using different modes

* PostGIS and lots of other great extensions

* supports most of the current SQL standard and is clear on interpretation of edge-cases in the documentation

* support for writing stored procedures in any major programming language

* many useful functions regarding dates, sets, ranges, json, xml, ...

* custom datatypes

* extremely thought-out and standardized approach to development: if a feature is included it generally works well in interaction with everything else

* syntax, semantics and performance are all very predictable

* great documentation

Regarding MySQL / MariaDB: MySQL optimized for performance first. Until 2010 the standard-storage-engine MyISAM didn't even support transactions.

PostgreSQL always focused on correctness and stability and then made sure everything performed.