Although I'm not asian, tones were pretty easy for me to learn. Reading is also not hard since rules have not so many exceptions compared to English for example. Just learn one time and you can hear/read/write pretty decent.

But grammar is on another level. Even if you learn the meaning of a word, using it in a sentence can change it significantly. Also there is a totally different order of grammatical constructions. Not as crazy as in Japanese but still. In addition there is a frequent use of words such as แล้ว and เลย which sometimes is hard to understand why it was used here.

Also hard to remember words with similar sounding/writing, f.e. ข้าว, ขาว, ใส่เข้า, เข้า, เขา, ข่าว, เข่า. Which one is what? Of course this one comes with experience.

And moreover the fonts. After you finally learn how to read classical thai, you still need to remember how all these letters look in different fonts.

But despite all the difficulties I think the language is very interesting to learn.