My number one reason for choosing MongoDB is replication that just works out of the box and doesn't require either a read lock or shutting down the master to set up a new slave.
That's my understanding too, but MongoDB hit the sweet-spot for us, Riak couldn't handle the raw queries-per-second we needed without requiring extra hardware ($$).
Serious question: What database requires read locks or shutting down the master for setting up a new slave? Is it MySQL?
Both sounds like really weird requirements for replication which defeats half the purpose of having it. PostgreSQL has never had any of these two problems. Still working on improving usability but with the addition and improvements of pg_basbackup I would say it is almost there. Hopefully 9.3 will get timeline switching to simplify failover.