Key-value caches include APC & Memcached and Key-value stores include Voldemart and SimpleDB, both of which are “eventually consistant” (though this is not a key-value store requirement.)

MogoDB however -like CouchDB- is a document store, which is a very different model. The document DB model gives you performance approaching what you might achieve with Key Value, but with object modeling and querying beyond what you can get from RDBMS.

This sophistication in object modeling can lead to a tighter coupling bw database and code, even as functions themselves become more loosely coupled— both within the limits of what CAP theorem allows.

MongoDB is one of the most exciting things happening in Drupal now, storing every field of an entity in its corresponding document as an AJAX-ready JSON (BSON) object. I have seen the future of fields– and it is Mongo.