When is a gratuity not a gratuity? When it changes into an expected component of the staff's compensation. I am of the school (and IMHO this represents the vast majority of world travelers) that believes gratuities come into play only when something extraordinary is done.

Perhaps if the unusual pay scale situation with waiters and bartenders did not exist in the US, more US travelers would understand that no gratuities are ever "required." Somehow, the waiter/ bartender 15% standard has migrated to all sorts of new places (including, in my experience just this weekend, at a candy shop, on a fast food counter, and in a clothing store).