{ $sortByCount: <arbitrary non-object expression, see 'Behavior' for why> }

> db.example.insert([
  {_id: 0, x: 1},
  {_id: 1, x: 2},
  {_id: 2, x: 1},
  {_id: 3, x: 0}
]);
> db.example.aggregate([{$sortByCount: "$x"}])
{_id: 1, count: 2}
{_id: 0, count: 1}
{_id: 2, count: 1}

// Example #2
> db.example.drop();
> db.example.insert([
  {_id: 0, x: 1.4},
  {_id: 1, x: 2.3},
  {_id: 2, x: 1.1},
  {_id: 3, x: 0.5}
]);
// Note this is an expression, so does not count as an "object" for the sake of erroring.
> db.example.aggregate([{$sortByCount: {$floor: "$x"}}])
{_id: 1, count: 2}
{_id: 0, count: 1}
{_id: 2, count: 1}

// Example #3
> db.example.aggregate([{$sortByCount: {field1: "$x", field2: "$y"}}])
Error!