Exactly. I can imagine that they thought they were being very clever using ORM + MySQL instead of writing SQL to run on a more mature RDBMS. Well, decisions like that often result in you painting yourself into a corner. There's a reason that MySQL is free yet people still pay for Oracle...
You're probably right. I've never understood why Postgres has such a small mindshare. Between Postgres and SQLite, there ought to be no ecological niche for MySQL to exist...
MySQL does (did, at least; Postgres has been improving) replication better. Replication is really important for high availability, which almost everyone needs.
The irony, of course, is that a single Postgres server is frequently (IME) more reliable than multiple MySQL servers even when the latter is setup for HA.
In addition, replication is the next thing where PostgreSQL is vastly improving (according to their road map).
Also note that if you want to trade some of the ACID properties for better performance and replication, the so-called NoSQL databases (CouchDB etc) seem to be a better trade-off than, say, MySQL with MyISAM instead of InnoDB.
