V8 is another important change that isn't really touted much in this release.
People use Hadoop because they have to, not because it is great. Isn't it time for a better alternative now that the toothpaste is out of the tube re: map/reduce?
Java doesn't even have hash literal syntax! Why would you ever want to query document oriented data with it as your language of expression?
It's also not relevant that SpiderMonkey is being replaced with V8.
Mongo's map/reduce is also just a toy, not at all comparable with Hadoop's and being deprecated in favour of the aggregation framework.
Also, there already are decent alternative map/reduce implementations. Disco is a good example, with a similar design to Hadoop.
You can also run Hadoop MapReduce over data in MongoDB directly (24 - 29): http://www.slideshare.net/spf13/mongodb-and-hadoop