Horrible dress sense: white sneakers and white socks, pleated trousers, phones on belts, shorts in inappropriate places, always too casual.
Overfamiliar too soon, too quick to tu or du. Overshare about themselves, family, ailments, life in general. We don't care.
Smile too much. It's creepy.
Not just too loud (and they are quite loud); have to talk about every single thing that pops into their head as if anyone else is interested. "Please notice me!" How can we not?
Culturally closed-minded, "why can't they _____ like we do at home?"
Difficult in restaurants: demand substitutions, sauces on side, ingredient changes.
Know-it-alls the moment they arrive in a new country for the first time, because of what they read in their travel book. Yet slow to catch on in real time once they're there.
Shallow, superficial, no real desire to know or experience more of the place or people they are experiencing.
In a rush, trying to cram 10 cities into their 8 day holiday with completely unreasonable travel logistics. Fearful of 'normal' transportation: trams, metro, buses, trains, low-cost airlines.
All this, there is a very small group of Americans with a lot of experience living overseas who are very fun and open, not as negative as we can be, considerate and good-hearted. But you have to get through a lot of the other ones to find them.