I'd be careful to compare the map/reduce in couch with the one in Riak. A lot of the time, map/reduce in CouchDB is used to create something similar to a "view"

I'd be careful to compare the map/reduce in couch with the one in Riak. A lot of the time, map/reduce in CouchDB is used to create something similar to a "view" in SQL. This is an incremental process and is automatically done for adding only changed documents to the result the next time it is called. In Riak, map/reduce doesn't have this fancy feature. A big plus for Riak is the fact that you can actually shard riak automatically. Couch has to be sharded manually which destroys map/reduce.