The fact that it is a well-documented language that has evolved over thousands of years with almost no external influence and is entrenched with thousands of years of cultural concepts that are distinctly unfamiliar to a majority of the western world. Many phrases used in Mandarin today date back millennia. Also something that many people don’t recognize is that a single character can embody many meanings depending on the context. It’s not as simple as memorizing the character because you have to know which meaning a character is representing within a particular context.
