^Mandarin's killer feature are the accents there are 4 or 5 ways to pronounce each sound, and there are many parent sounds (around 40 I think). Many sounds are not distinguishable in my ears but make a huge different for them.
Second most killer (if you ever manage to speak) are the letters. Though there are more than 80,000 of them, they still use the same letter (and sound) for many different meanings, sometimes as much as 10 with completely different meanings for each letter.
And if this was not enough many times two letters are combined into one, with a different sound and meaning.
In a way it's a primitive language: no articles, inclinations, tenses, just rigid letters like lego. On the other hand each letter is a symbol and this can be extremely rich in other forms of expression such as calligraphy or poetry. Calligraphy is a big thing in China. And pretty much everyone (in my eyes) writes in a very controlled way, producing beautiful and well defined strokes. Quite impressive to see.
There is no way to know the sound of a word if you haven't memorised it before. You need to know ~15,000 -20,000 characters i.e. University degree.
As a result of a completely different language structure their education (too much memorisation) and the way they think are pretty different than us in Greek -Latin language family. I see similarities in behaviour in Koreans (had Chinese letters until recently) and Japanese (they still do). Typically they don't challenge the authority as we do, authority meaning your line manager, the elder anything. They don't say plain no as we do. Nevertheless there is some depth and wisdom in their ways.