List of new features in 9.3
https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_...
https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_...
Is there any context in which it would make sense to set up circular replication? This would necessitate that all of your nodes be read-only, so I'm not sure what there would be to replicate.
In any case, a bunch of these new features are hot. Particularly excited about fast failover and custom background workers.
It depends on if each node in the loop knows that it's already seen the replication data before and thus logs and ignores it. That's basically how MySQL (multi-master) replication loops work.