Stranger Small Talk
There's nothing worse than making small talk while in line or on the bus or some other place where you'd rather just be left alone. Again, Americans just really love to talk, and even if the other person isn't super receptive to a conversation, it doesn't matter. In other countries, people value their alone time while out and about.
Pajamas in Public
Americans, no matter how busy they are, still think it's acceptable to wear pajama pants while running quick errands. To foreigners, this is completely weird. There's a reason why most "People of Walmart" are in their PJs.