• Read-replicas are deployed in a single Availability Zone; there is no standby read-replica.
• It's not possible to enable backups on read-replicas during time of creation. This must be configured afterwards.
• The default storage type is standard (magnetic). You can increase performance by choosing gp2 or using provisioned IOPS.
• It's possible to add MySQL indexes directly to a read-replica to further increase read performance. These indexes are not required to be present on the primary DB.
• Using read-replicas for availability purposes is more of a complimentary DR strategy and shouldn't be used in place of multi-AZ RDS. A multi-AZ configuration gives you the benefit of failure detection and automatic failover.
• It is possible to deploy a read-replica in an entirely different region.
• Unlike the replication between a primary and standby DB (which is synchronous), replication to a read-replica is asynchronous. This means that it's possible for a read-replica to fall behind the primary. Keep this in mind when sending time sensitive read queries to your read-replicas.