Can someone review a short script and tell me where I've gone wrong? [JavaScript]

I have began coding a controller for a 2D game and I am having a few issues. Could anyone help me out please! alt text

Code

#pragma strict

	var moveUp : KeyCode;
	var moveDown : KeyCode;
	var moveRight : KeyCode;
	var moveLeft : KeyCode;
	
	var speed : float = 10;
	var jumpHeight = 8;
	
function Update () 
{
	if (Input.GetKey(moveUp) && isFalling == false)
	{
		rigidbody2D.velocity.y = jumpHeight;
	}
	isFalling = true;
	
	else if (Input.GetKey(moveDown))
	{
		rigidbody2D.velocity.y = speed *-1;
	}
	else if (Input.GetKey(moveRight))
	{
		rigidbody2D.velocity.x = speed;
	}
	else if (Input.GetKey(moveLeft))
	{
		rigidbody2D.velocity.x = speed *-1;
	}
	else 
	{
		rigidbody2D.velocity.y = 0;
		rigidbody2D.velocity.x = 0;
	}
}

check the script reference. It is one of the biggest assets of the Unity engine, and it quickly improves your understanding on basic scripting stuff