JSON falls flat when trying to define data types and serialize objects.

  var foo1 = new Foo1();
  foo1.bar1 = new Bar1();
  var foo2 = new Foo2();
  foo2.bar2 = new Bar2();

If you were to serialize that data using XML you might do this:

  <Foo1 name="foo1">
    <Bar1 name="bar1"/>
  </Foo1>
  <Foo2 name="foo2">
    <Bar2 name="bar2"/>
  </Foo2>

But now what if you use JSON? You'll probably end up with something like this:

  {
    "foo1" : {
      "class" : "Foo1",
       "@bar1" : {"class":"Bar1"}
     },
    "foo2" : {
      "class" : "Foo2",
      "@bar2" : {"class":"Bar2"}
    }
  }

Generally it makes sense to use XML for imperative data structures, and use JSON for functional data structures.