MongoDB uses btree indexes as well, it just doesn't use Map/Reduce for its regular querying like CouchDB does. See my above comment on differences as well as a writeup I did on new output options in MongoDB's M/R system.
The original part in couchdb design is using btree for views to hold temporary data for M/R jobs, as btree are indeed used by mongodb (and most db engines).
The new stuff for M/R in mongodb looks interesting, thanks for the update.
