That would probably be the necessity of using completely different language (vocabulary, grammar, final particles, etc.) depending on your conversation partner's age, gender, and other such considerations.

the different meanings of kanji between Chinese and Japanese

The sarcasm of Nihongo Jozu

Learning Kanji was the most challenging part for me. What makes it hard is that the character and its pronunciation are unrelated. However, that is not enough to stop me, it is a beautiful language 🤍