The way I understood it, Diet Coke was a Diet version of the New Coke formula. Remember that one?
And now Coke Zero is a diet version of Coke Classic.
I think I was reading that off of the coke website forum. I had to research this as after I tasted it, I didn't believe it was diet. It tasted just like real Coke to me. Of course, I've only drank diet cokes for years, so I don't notice aftertaste anymore.
At any rate, for me, it used to be DC w/ Lime, but now I'm Zero all the way.