I think schema-less plus excellent performance out of the box give MongoDB an edge. You also don't want to discard advance features like map/reduce buit-in in J

I think schema-less plus excellent performance out of the box give MongoDB an edge. You also don't want to discard advance features like map/reduce buit-in in JS, chaining of commands, ability to have nested objects (think tables inside tables), ability to do atom modifictations to objects, etc.