The boundary between "array" and "object" is particularly blurry for some languages (like PHP). It can be very easy to accidentally write something out, thinking it is an array, only to have it actually be stored internally as an "object".

This wouldn't be a problem if array operators (such as $elemMatch, $in, $all, $slice, $size, $push, $pushAll, $addToSet, $pop, $pull, $pullAll) worked on objects. Also $insert and $remove from SERVER-2036 if/when they are implemented.