I don't even know how to define a single culture. The best way that I heard is: A member of your culture is someone you don't need to explain a joke to. That makes culture fluid and you could be a member of several different ones (which I like). It also means that the US is not even close to a single culture and that is absolutely fine.
There is also a problem with a list that it doesn't tell you how to define a term. American Exceptionalism can go in very different ways.
I admit when trying to think of the list I tried to think of things that are in the air of conversation but are either ill-defined, never explicit, or just not mentioned enough (for my taste):
4th of July, Human Rights, Depression (economic and personal), Fascism, Communism, Middle Passage, Manifest Destiny (and Trail of Tears), World War (I & II), Rights (absolute, non-absolute, restricted) Citizenship, PTSD.
Admittedly, that is eleven(ish). And also it came to be a bit more political than I wanted it to be. Also both the article and I fail at mentioning that Hirsch added terms from the sciences (I don't have the book on me, but I remember things like Electron on the list). But in all honesty, I don't want the list to exist. Common core is a mess. No Child Left Behind is a pain to every college instructor (I am one of them, it is a problem). I don't think a list can or should exist. I much prefer a better education system that will allow people to find things that they are looking for rather than have a checklist for the establishment to tick off.