The traditional method for formulating a diet beverage is to take the normal version, strip out the sugars, and replace them with calorie free sweeteners. After doing this, the flavor is tinkered with a little, and the product is released. This is of course the wrong approach and has created a large number of vile concoctions.
Diet Coke, on the other hand, was created from the ground up. The result, if its rapid rise to the third most popular soda is any indication, is a diet cola that actually tastes fairly good. Aside from the faintly unpleasant aftertaste of the aspartame, Diet Coke is a legitimately tasty cola in its own right.
Interestingly, when Coca-Cola was creating New Coke, they reversed the traditional diet cola formulation process. They took the hugely popular Diet Coke, stripped out the aspartame, and added high fructose corn syrup. Taste tests showed this to be a very promising product, the failure of which is now credited largely to marketing ineptitude and not a lousy flavor in and of itself.
Culled from snopes.com's article on whether or not New Coke was just a sophisticated marketing ploy.