Postgres won't let you perform any sufficiently long (read-only) queries on slaves. For example, I was unable to run pg_dumpall on a slave to have DB snapshots.

This is [well-explained](http://www.postgresql.org/message-id/201102272005.00234.jens...), but still inconvenient. This way, slaves are only useful for failover and possibly offloading very short read-only transactions, which is pretty limited.