If It Ain't Broke, Don't Extend it
An axiom of open-source software is that developers write it to "scratch an itch." This is a good practice when the itch is something like a failing disk, which compels the developer to improve the tools that work on the disk. However, in regards to databases, the itch flares up only when data surpasses certain limits, such as size or complexity. PostgreSQL and MySQL boast widespread use for relatively small databases (under 100GB, for example). Once the data grows larger than 100GB or so, the number of users drops off drastically. At that point, working through large-database-related issues becomes more of a problem.
The itch axiom also applies when working in some of the more "buzzword-complaint" areas. Some of the more advanced features in the open-source databases (such as replication) are nowhere near what you'd find on commercial alternatives. Quite simply, most users don't need replication at the levels that Oracle, DB2, or MS-SQL offer; therefore, PostgreSQL and MySQL developers don't get the itch to improve it.
The great thing about open source software, though, is that it's pretty easy to try out and has lots of freely available online documentation to help you learn the products. While these databases may not be optimal for every project, they work very well for others. If you're curiousand this article hasn't answered your particular usage questions, take MySQL or PostgreSQL out for a spin and see if they meet your needs.