Japanese: topic instead of subject marking, deference forms, completely different morphological categories?
Chinese: productive verb serialization, very different syntax?
That's just off the top of my head
But that's not the point really. Learning any language well is mind-bending for any speaker of another language. It does help if the languages are different (I remember very vividly the click that went in my head in high school when I was taking English, Chinese and Hebrew simultaneously... that's how I ended up in linguistics grad school), but different people have their minds bent by completely different things. I mean, root-and-pattern morphology is really cool when it's that regular, but if you think of it it's not horribly different from Germanic strong verbs (write-wrote-written). Weird agreement patterns? Well, Welsh only uses the 3pl verb forms with a pronominal subject, so "they they-went" but "people he-went" (as, for that matter, do some varieties of English). And so on.
I mean, sure, whatever floats your boat, and it's awesome that studying Arabic can be so fulfilling; I've had the same experience with Welsh. But at the end of the day all languages bend your mind, and I don't think there can be an objective metric. They are just hard.
Japanese: topic instead of subject marking, deference forms, completely different morphological categories?
Chinese: productive verb serialization, very different syntax?
That's just off the top of my head
But that's not the point really. Learning any language well is mind-bending for any speaker of another language. It does help if the languages are different (I remember very vividly the click that went in my head in high school when I was taking English, Chinese and Hebrew simultaneously... that's how I ended up in linguistics grad school), but different people have their minds bent by completely different things. I mean, root-and-pattern morphology is really cool when it's that regular, but if you think of it it's not horribly different from Germanic strong verbs (write-wrote-written). Weird agreement patterns? Well, Welsh only uses the 3pl verb forms with a pronominal subject, so "they they-went" but "people he-went" (as, for that matter, do some varieties of English). And so on.
I mean, sure, whatever floats your boat, and it's awesome that studying Arabic can be so fulfilling; I've had the same experience with Welsh. But at the end of the day all languages bend your mind, and I don't think there can be an objective metric. They are just hard.