To be clear, "replication" in this case is "multi-master replication."

Riak's Replication: http://docs.basho.com/riak/1.3.2/references/appendices/conce...

CouchDB's Replication: http://docs.couchdb.org/en/latest/replication/intro.html

Riak's is for data safety within the cluster.

CouchDB's is multi-master, cross-cluster, cross-device, etc. It can either be uni-directional (push or pull) or (if you do both) it can serve to synchronize two distributed master databases--each serving as a primary write point in the architecture.

That seems unique.