Each ancient civilization in the Americas had unique features that distinguished them from one another. However, if you are looking for a singular unique aspect that is often highlighted, it's generally considered to be "its writing system." For example:
- The Maya developed a complex hieroglyphic writing system.
- The Aztecs had pictograms and ideograms, although theirs was less extensive than that of the Maya.
In contrast, many other civilizations, such as the Inca, did not have a formal writing system but used quipus (knotted strings) for record-keeping.
So, the response focusing on "its writing system" encapsulates a unique feature that varied between civilizations.