works:

cursor=db.points.aggregate([{"$group":{"_id": "$created.user", "count":{"$sum":1}}},{"$sort":{"count":-1}}])


does not work:

cursor2=db.points.aggregate([{"$match":{"amenity":{"$exists":1}}}, {"$group":{"_id":"$amenity","count":{"$sum":1}}}, {"$sort":{"count":­1}}, {"$limit":10}])