It doesn't have to be, but if the only thing uniting them is agreement on a common enemy, then everything I know about human nature says that it's going to end

It doesn't have to be, but if the only thing uniting them is agreement on a common enemy, then everything I know about human nature says that it's going to end up that way. I would love to read a detailed, informed comparison of CouchDB, MongoDB, Tokyo Tyrant, Redis, etc. by somebody who knows enough about RDBMSs to realize they don't all suffer from MySQL's limitations. I'm curious what use cases they fit the best, etc., but most things I've read have only been familiar with one or two of them (understandable), or has been very heavily biased towards somebody's pet project or new favorite.