The $maxDistance operator is misplaced in the documentation for the $near operator. The syntax and GeoJSON example puts the $maxDistance operator inside of the $near document instead of within the $geometry document as it should be. Either placement works fine for MongoDB 2.4.x, but the syntax is more strict for >= 2.5. I believe the example should be:

db.places.find( { loc : { $near :
                          { $geometry :
                            { type : "Point" ,
                              coordinates: [ 40 , 5 ] }
                            $maxDistance : 500
                          },
                        }
                } )