I am wondering whether map reduce job in MongoDB has anything to do with Hadoop. Mapreduce in MongoDB is a standalone and no dependency on any hadoop installation? If what I am guessing is correct, then the map reduce syntax is the same between the two or it just means that MongoDB is supporting its own map reduce (with different syntax)?
Map Reduce is not fastest interface in MongoDB to do ad hoc queries it is more designed for background jobs, creating reports etc. I wrote some time ago how to do it on my blog