I think a lot of the problems you hear with MySQL come from people who are experienced in more robust enterprise level RDBMS. Below are some things I lifted from a website comparing SQL Server and MySQL.
* MySQL does not support Triggers, SQL Server does.
* MySQL does not support User Defined Functions, SQL Server does.
* MySQL does not have Cursor Support, SQL Server does.
* MySQL does not support Stored Procedures, SQL Server does.
* MySQL does not have FULL JOIN capabilities, SQL Server does.
* MySQL has limited Import/Export capabilities, SQL Server has full support for Import/Export.
* MySQL has limited Transaction Support while SQL Server offers full Transaction Support.
From what I've read it sounds like PostgreSQL has more of these features that make it more of a "real" RDBMS. Had people in the PHP community adopted PostgreSQL over MySQL I don't think you'd really hear much about MySQL. It's an easy DB to install, it's widely available on web hosts, it's got a lot of free tools (PHPMyAdmin) that make it easy to "administer" and it's free and easy to get.
I would LIKE to learn more about PostgreSQL but I don't see it as something that's instrumental in my development as a... developer.