Simplest transparent surface shader

I'm trying to learn more about surface shaders. Right now I'm trying to mimic the behavior of the standard transparent shader, but I can't get the transparency to work as desired. Even when the alpha is set to zero, there's still a strange gray being rendered.

Code (CSharp):

1. Shader "Custom/NewShader2" {
2.     Properties {
3.         _Color ("Color", Color) = (1,1,1,1)
4.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
5.         _Glossiness ("Smoothness", Range(0,1)) = 0.5
6.         _Metallic ("Metallic", Range(0,1)) = 0.0
7.     }
8.     SubShader {
9.         Tags {"Queue"="Transparent" "RenderType"="Transparent"}
10.         LOD 200
11.        
12.         CGPROGRAM
13.         // Physically based Standard lighting model, and enable shadows on all light types
14.         #pragma surface surf Standard alpha
15.  
16.         // Use shader model 3.0 target, to get nicer looking lighting
17.         #pragma target 3.0
18.  
19.         sampler2D _MainTex;
20.  
21.         struct Input {
22.             float2 uv_MainTex;
23.         };
24.  
25.         half _Glossiness;
26.         half _Metallic;
27.         fixed4 _Color;
28.  
29.         void surf (Input IN, inout SurfaceOutputStandard o) {
30.             // Albedo comes from a texture tinted by color
31.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
32.             o.Albedo = c.rgb;
33.             // Metallic and smoothness come from slider variables
34.             o.Metallic = _Metallic;
35.             o.Smoothness = _Glossiness;
36.             o.Alpha = c.a;
37.         }
38.         ENDCG
39.     }
40.     FallBack "Diffuse"
41. }

What exactly is required to get it to behave as the default standard transparent shader?

 

Why mimic it if you can just look at the source:
http://forum.unity3d.com/threads/builtin-unity-shaders-source.2085/page-2