As for American vs. Australian vs. European Chocolate - namely Cadbury...
I actually did sort of a side-by-side taste test when I got back from Australia two summers ago. -Remember my one 17-pound carry-on that only held chocolate?!?!?

Well...
I think it just comes down to the base ingredients - mainly the sugar and milk. I could sense the general formulas were adjusted according to local tastes. However, I could also sense that the Australian products used better milk. And better sugar. Things tasted purer, less processed. Environmental conditions, I guess;less pollution.
Australian and English Cadbury are definitely less sweet than American Cadbury - and almost any American mass-produced chocolate. And since there's less sugar in their recipes, the chocolate is creamier, has a better mouth feel. -Of course, there's the Crunchy and Violet Crumble which are chocolate covered "honeycomb" candy bars, and the whole point of those is "sweetness"... But I digress...
And after reading labels, the English and Australian chocolates use less - if any - preservatives and stabilizers.
So, that's my very unscientific analysis...
Hmmm... Wonder if I can make a run to the corner store...