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.