Javascript and C#, different behaviour in inspector ?

Hello everybody ^-^ and Happy New Year :D (few days late ^-^')

I've recently changed from javascript to c# and i have a slightly doubt about the inspector behaviour with the last one.

I mean, when i add a class as a variable of another in javascript:

 #pragma strict
 
 public class A extends MonoBehaviour {
     public var pol : int;
     public var b : B;
 }
 
 #pragma strict
 
 public class B {
     public var al : int;
 }

in the inspector i can access the public variables from the second (B) in the object containing the first (A), like this:

But when i try to do the same in C#:

 using UnityEngine;
 using System.Collections;
 
 public class C : MonoBehaviour {
     public int pol;
     public D d;
 }
 
 using UnityEngine;
 using System.Collections;
 
 public class D  {
     public int al;
 }
 

I can't get the same inspector menu (i get this):

Unless i use [System.Serializable] in the second :

 using UnityEngine;
 using System.Collections;
 
 [System.Serializable]
 public class D  {
     public int al;
 }

Then i get the same result as in javascript (in the inspector C has the D menu to modify, can't post more images it seems ^-^').

So my question is, am i doing something wrong in C# to have that different behaviour? Is that javascript is already serializable (has something similar to System.Serializable by default in each class already)? Should i change the way of doing it? :o

Thank you in advance ^-^