1) You need to know a few thousand Kanji to be ‘literate’
Newspapers require over 1000 at least, while you require around 2000 to be able to pass the JLPT N1. Im sure more specialized fields (science, technology, etc) will have all of their own bunch of specialized kanji, I dont even want to know how many you’d need to be a pharmacist, doctor or author.
2) All of the Kanji look the goddamn same
Examples (and there are many):
持 時 寺 待
目 貝 見 買
The meanings of all of these characters are, of course, completely different. You might not think it’s too bad, but believe me once you have a few hundred of these under your belt, everything really does start to look identical. If you are lucky, context will help the Kanji make sense, but if you are unlucky enough to get 2 characters confused which are even remotely interchangeable (Verbs are a great example) you’re gonna have a confusing time.
3) The pronunciation of Kanji changes depending on its context
The only thing more fun than learning how to read a Kanji? learning how to read it 4 times, and then having to remember them all when it actually comes to reading it.
A quick example: the character for say or speak – 言 – has at least 4 readings
言う – “Iu” – ‘i’ – ‘to say
言葉 – “kotoba” – ‘koto’ – a word
言語 – “Gengo” – ‘gen‘ – language
伝言 – “Dengon” – ‘gon’ – message
So straight away we can see that there are at least those 4 readings (I / koto / gen / gon) depending on the context it is used in.
Almost all of the characters have at least 2, which is great fun for learners (not).
4) They don’t use spaces between words
5) Many readings don’t even make sense
For example, ‘Seinen’ 青年 which means Young Man – is made out of the characters 青 (which means Blue) and 年 (which means Year).
青年 ＝ 青 ＋ 年
YOUNG MAN = BLUE + YEAR
These sort of readings are, thankfully not particularly common, and yet there are enough of them that you will be left utterly clueless if you do not know your kanji perfectly, this is because (apparently) Japanese/Chinese are logographic languages – the jist of which is that they use symbols which represent words rather than words which represent sounds (phonogramic languages).
Admittedly there are advantages to the logographic style used in Kanji, for example I can see something I can’t quite read and be able to maybe tell what it means, but I still think I’d rather a language which doesn’t involve literally thousands of characters in its script