From everything I've read about MariaDB, its supposed to be a drop-in replacement.
Performance comparison: http://slashdot.org/topic/bi/mariadb-vs-mysql-a-comparison/
There have been a number of problems with Oracle taking over MySQL--top among them is that it's no longer fully open source and Oracle has effectively shut out the development community.
And, probably worst of all, it's impossible for the community to work with the MySQL developers at Oracle.
- Oracle doesn't accept patches
- There is no public roadmap
- There is no way to discuss with MySQL developers how to implement things or how the current code works.
I used PostgreSQL several years ago, and while I think it's a very good DB engine and doesn't take a lot of the sloppy shortcuts that MySQL does, unfortunately, it simply hasn't caught on. MySQL just happened to come out at exactly the right time, an no other database has been able to gain a foothold like it did.
Because of MySQL's popularity, it means is that a lot of webapps are specifically designed for the LAMP/WAMP stack, and there are no considerations taken for running on alternate databases. PostgreSQL is not a drop-in replacement for MySQL. (and yes, I've been guilty of also designing for the LAMP/WAMP stack and not including support for other databases...the stack simply is just deeply entrenched in web development and server hosting environments).