"Views only operate on the original pool of documents, and are not able to be a view of views."

The inability to chain map/reduce calls is the biggest thing holding CouchDB back, particularly w.r.t. CouchApps that don't have the luxury of an easy workaround. There is a bug https://issues.apache.org/jira/browse/COUCHDB-249 however no progress has been made recently.

By allowing views on the output of other views, more "normalized" document structures could exist that would still be transformable into useful outputs.