That is, until, we travel abroad and experience culture shock after observing how others interact with each other outside of our own country.
"What's an incredibly American thing americans don't realize is american?"

Americans respond differently than people from other nations when it comes to indicating where they're from.
Location Location Location
"I used to work in a call Center that took calls from Americans and Canadians. I’d ask 'Where are you from, Canada or the US?' Canadians would say 'Canada' and American would say 'Texas' or 'New York' etc. Never ever would they reply with their country name."
– fancyllamapants-
Sticking To Specifics
"Might just be me, but I do notice when you ask Americans on the internet where they're from, they reply with either a state or a city instead of their country."
"The thing is, it doesn't cause any confusion, since most people know most American states and at least the major cities."
"You don't often see an Indonesian person, for example, say they're from West Java. Just that they're from Indonesia."
– FlygonsGonnaFly
More Background
"I also find that Americans often include where they were born. For example "I'm from Philly but originally from Jersey.'"
"Edit: I understand that the US is big and has many different regions. I'm Canadian and find that Canadians typically don't answer that way, despite being very large."
– RedSpikeyThing
Some people are dumbfounded by these the following behaviors and tendencies.
Shortened Pronunciation
"Its the American pronunciation of 'squirrel' that gets me. Here its a two syllable word. 'Sqwe-rell' kinda. But when my American gf says it it comes out kinda like 'sqwurl.' The first time she said it I thought she said 'swirl'. Its just one syllable."
– DonSmo
Favored Flavor
"Ranch. I never knew ranch was just an American thing until recently."
– BlocBoyBaloo
Time To Close Out The Check
"Not having to ask for the bill."
– Ok_Independence_5025
Complete Trust
"Handing your credit card to a stranger, having them walk away, swipe it, then bring it back to like they didn’t just put a down payment on a new house with it…"
– browncoat47
European Academics
"Universities in Europe are simply institutions of higher learning, nothing more."
"Americans would be surprised by the cultural differences when it comes to university education. There are no athletic teams, no Greek life, no 30 foot climbing walls at universities in Europe. You come to school to learn and leave when your classes are over for the day. If you want to study, you can go to the library, but there is very little that a university offers outside of academia."
– sharksedition
Simple Dish
"Mixing three different canned foods together and calling it a casserole."
– calls_you_a_bellend
Transactions that seem normal to Americans is considered completely unusual in other countries.
Name The Price
"Not including tax on prices displayed in stores."
– rubs_tshirts
Always On-The-Go
"Drive thru atms and everything else."
"I didn't learn we had drive thru liquor stores until later in my life."
– cbeiser
Side Effects May Include Feeling Jaded
"Prescription drug commercials."
– Dhk3rd
"Me and my husband love watching the American feed of NFL. We quite like the drug ads, as they have to list all the side effects and it sound like every drug will kill you. We love laughing at the ads."
– toonlass91
Tipping has always been a practice I never understood.
In many other countries, it is not customary to tip because–from what I understand–service industry employees are already incentivized to work hard and provide excellent service because they are already more than reasonably compensated.
Depending on the situation, I think tipping is a flawed system that doesn't work fairly for the consumer and the employee as it can breed resentment.
But that's just my two cents -- which for the record is NOT an appropriate tip. The figure you're looking for is 20%.