interestingasfuck

You can learn up to early intermediate Japanese pretty easily because you don't have many inflections to worry about--there are no plurals, no verbs changing according to person (I vs he vs they), no articles, etc. And the verb conjugation is very rigid with literally only two irregular verbs. Also the pronunciation is pretty easy coming from English.

If I had to think about the one aspect that makes it so difficult to continue progressing it'd probably be that not being able to read is a huge roadblock to a major form of language acquisition. That plus a limited amount of sounds makes tons of words homophones or similar. For example you've got ko-kyu-u (breathing) vs ko-oh-kyu-u (high quality, high salary, or permanence depending on how you write it). Yeah they have different pitches (kind of like tones but not really), but that's just another thing you've gotta remember.