The documentation for $all says:

The $all operator selects the documents where the value of a field is an array that contains all the specified elements. 

However this does not match the following behavior on master:

> coll.drop()
> coll.insert({_id: 1, a: 1})
> coll.find({a: {$all: [1]}})
{_id: 1, a: 1 }

If these are the intended semantics, we should update this ticket to request a docs update. If however, these semantics are unintentional, and $all should only match arrays (similar to $elemMatch), this ticket should fix the behavior.