What American habits stand out in Italy?

Volume, mostly. Americans speak at a level that carries across an Italian restaurant, and it's the number one thing locals notice. After that: asking for the check the moment you finish eating, which reads as being rushed rather than efficient — in Italy you ask for il conto when you want it, and it will never arrive unprompted. Also common: eating dinner at 6:30, wearing gym clothes and baseball caps to dinner, ordering a cappuccino after a meal, and tipping 20%. None of these are offences. They're just tells, and dropping them makes you a guest rather than a tourist.