Fleshing out the Feb 30 comment a little more....
The problem is in the philosophy regarding data vs application in the two products. MySQL treats the data as king and allows any application to turn off safety measures. PostgreSQL treats data constraints as king and expects apps to honor them.
This means that the two aren't really competitors any more than either is a competitor to SQLite. MySQL is a fine choice when you want a dumb store for your data and trust the application to do all necessary checks. PostgreSQL is a lot more pedantic and that makes it better when you want multiple front-ends to the same data.
The only real problem with PostgreSQL replication is the plethora of choices surrounding it.