Respect

Italians are generally open and friendly, if you use the regular politeness you will have no problem.
Italians, especially those in the North, are very different from the stereotype of "pulcinella, pizza and mandolino" seen in American "B" movies (and you won't find roads full of Fiat 500's). Not surprisingly, for many people this stereotype is quite offensive.
During WWII, Italians had a difficult time under the dictator Benito Mussolini, especially after the infamous alliance with Adolf Hitler fell and the Germans turned into enemies. After 60 years this still is a very sensitive subject and you should simply avoid it, unless you want to seriously discuss. Avoid jokes on the subject too.
Italy has a high population of senior citizens, and it is wise to be respectful. Do things like offering your seat on a bus if the situation arises.