Everything lol. The tones are hard for beginners, the sheer amount of characters you need to learn can be very intimidating, Chinese grammar is very difficult to really get your head around, you need to learn almost every single word from scratch which can be very demotivating as learning vocabulary is difficult and time consuming, the fact that spoken Chinese and written Chinese are totally different is also very confusing and can be annoying because you can't read things you would otherwise understand if someone said them to you.

For me, it was the fact that neither Hanyu Pinyin not Zhuyin Fuhao are 100% phonetic. I had to watch Fluent Forever's videos on Mandarin phonetics several times, learn the IPA symbols for the sounds that caused me trouble and practice lots of times with native recordings. I also needed some "ear training" to help me distinguish between -N and -NG, and I still sometimes mess up. So, for beginners, my advice would be: When in doubt, check the IPA and native recordings. Do some serious ear training and keep working on each sound until you nail it. It's like trying to pass a difficult level in a videogame.

Chinese gets a lot easier once you reach an elementary level, but getting past the initial hurdle of tones, pinyin, and all those characters.....it’s a lot to wrap your head around as a beginner. Even though the rules aren’t necessarily any more complicated than any other language, a lot of them are very different, and Chinese pronunciation is relatively unforgiving of mistakes. As an advanced learner, the big difference in written and spoken Chinese, the presence of remnants from Classical Chinese in modern idioms, poetry, and fiction are all big challenges.