This is a great demonstration of the fundamental issue with node. His code goes from:

   function insertCollection(collection) {
     for(var i = 0; i < collection.length; i++) {
       db.insert(collection[i]);
     }
   }

To this:

  function insertCollection(collection, callback) {
    var coll = collection.slice(0); // clone collection
    (function insertOne() {
      var record = coll.splice(0, 1)[0];
      try {
        db.insert(record, function(err) {
          if (err) { callback(err); return }
          if (coll.length == 0) {
            callback();
          } else {
            insertOne();
          }
        }
      } catch (exception) {
        callback(exception);
      }
    })();
  }

Hopefully this article will work as an eye-opener for anyone who was not yet convinced that node badly needs a concurrency abstraction.