Postgres replication is great, but it does have flaws.

The big one is that it's on a per-cluster (ie., database instance) level. It's not possible to have different databases with different replication settings: You have to replicate everything or nothing.

Another gripe is that it's awkward to set up the first time; you have to do a base backup, rsync over, etc. It would have been great if you could just start a slave and tell it to stream the entire master database over. Possibly something that gets easier in 9.3.

Another gripe, as a developer, is that read-only queries can fail. You will eventually get a nice "ERROR: Canceling statement due to conflict with recovery"; and you will simply need to retry the query at that point. (We actually switch back to the master and retry.) We use long timeouts for the pertinent settings (see http://www.postgresql.org/docs/9.2/static/hot-standby.html), but we still get these.

Some MySQL fans would probably say that Postgres replication being single-master/multiple-slave is a problem, but I don't mind myself.