Why aren't some variables appearing in the Inspector?

I just added a public variable with the type String to a script. However the variable is not appearing in the Inspector. The same goes for other types such as boolean,float,int. Is this a bug or am I doing something wrong?

This is a javascript script

var quitButton: boolean = false;
var normalTexture : Texture2D;
var rollOverTexture : Texture2D;
var levelToLoad : String;
var beep : AudioClip;

I have actually found that the boolean and String will be visible and assignable once I have attached it to an object. I guess the question is solved but I would like to know why the Texture and Audioclip can be assigned even when the script is not attached to any object?

To answer your edit - you can edit the default values in the script for string and boolean, but you can't do the same for audio clip, textures and the like, so unity provides a way to set up default references which will be set when you add the script to a gameobject, just as the default values in the script will be set for booleans and strings