The original similarity design theme was started was Bruno Sacco who coined the designs for W124, W210, W126 and W140. He said a mercedes should look like a mercedes and all its models should be identifyable without having to look at the name. Something about verticals and horizontals.
That is why you can ask any common person to identify them and they know its MB, the 201, 124 and 140 look like they went from small to big of the same model.
http://en.wikipedia.org/wiki/Bruno_Sacco