For the inputs on the Material node
Add >> Input >> RGB (String RGB Color Output to Color Input)
Add >> Input >> Value (String Value Output to Spec Input)
Add >> Input >> Value (String Value Output to DiffuseIntensity)
The combinations of these three can mimic and exceed the capabilities of the property sliders.
To Start with Set RGB to R:0.8,G:0.8, B:0.8
For Spec set the value to: 0.8
For DiffuseIntesity set the value to: 0.8
These values should get you pretty close (if not exact to the default material).
Without Nodes: Specular Intensity Controls the main "Light" bounce (Size/Intensity).
With Nodes: The Value of Spec correlates to "Specular Intensity"
To demonstrate this: with use nodes disabled, set Specular Intensity to 0.8.
Now toggle (enable / disable) use nodes to see the difference.
Now in the node editor change the Spec value to 1.5, now toggle again.
With use node disabled set Hardness to 40.
With use nodes enabled in the node editor
Do the following:
Set color (R:0.55,G:0.55,B:0.55)
Set Spec: 1.8
Set DiffuseIntensity: 0.7
This isn't perfect but it will be close.
The whole reason for going through the exercise is to show how NON-Intuituve these parameters are to set on their own.
You can get there, but there are parameters to consider that the property sliders just make way easy for the user.