With the caveat that I've only travelled in the Western world, credit cards really are almost universally accepted, Germany aside. Obviously, the smaller the merchant, and the smaller the country/town, the less likely.
As an American, a particularly odd experience was at a shop in Peru, where they only accepted MasterCard and not Visa. These days we're so used to them being interchangeable, it had never even crossed my mind. Between my wife and I, we had 5 or 6 Visa debit/credit cards, and 0 MasterCard.
