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：漢語語調稀奇古怪。
在學漢語過程中，最容易聽到的抱怨正是這一點，西方人對漢語最害怕的也是這一點，簡直是婦孺皆知。如果不是從小就會說漢語，那你就等著不停地去死記那些元音輔音吧。真正的難題還要等你真的要用漢語進行表達的時候才會出現。漢語的那些語調和強調習慣簡直是與生俱來的，根深蒂固的，簡直不可思議。