Top 6. Because tonal languages are weird.
It's one of the most common complaints about learning Chinese, and it's also one of the aspects of the language that westerners are notoriously bad at. As non-native speakers, you must memorize along with the vowels and consonants. The real difficulty comes in when you start to really use Chinese to express yourself. Intonation and stress habits are incredibly ingrained and second-nature.

原因六：汉语语调稀奇古怪。
在学汉语过程中，最容易听到的抱怨正是这一点，西方人对汉语最害怕的也是这一点，简直是妇孺皆知。如果不是从小就会说汉语，那你就等着不停地去死记那些元音辅音吧。真正的难题还要等你真的要用汉语进行表达的时候才会出现。汉语的那些语调和强调习惯简直是与生俱来的，根深蒂固的，简直不可思议。