Actually the Coca-Cola doesn't differ across countries, it's the way of storing and serving which makes the difference in taste (f.e. at McDonald's). Other Coca-Cola brands such as Fanta are different in several countries, due to a different composition.
The composition of Fanta, for the same flavor, varies from country to country. For example, the European Fanta Orange contains orange juice (in varying percentages but typically 6%), whereas the US formulation does not . The Australian version is 2.1% fruit juice, and South American formulations also have orange juice, especially in Brazil, where Fanta is 10% orange juice. These differences mean the taste of Fanta differs greatly from country to country, more so than regular Coca Cola, and may in part explain why the drink's popularity varies so much between different countries. Difference in coloring can be observed internationally. For example, where European Fanta Orange is light orange, almost yellow, in Latin and North American markets it is deep orange in color.