While there are good JP and NA players remember there are selfish JP and NA players.
Personally one thing I hated about NA players looking from JP players point of view are those who comes to EXP party and once they level they just F'ing leave, or once it's their time they just leave. In JP side they had some MMO manners or w/e they came up with from other games where they let others know they have to leave in X minutes, normally being 15+ min (unless emergency) or they will wait till the replacement comes. Some asshole players will just leave the party even without waiting which causes rest of the party to need to wait for the missing spot in the party to be filled. (Although in later FFXI updates it became so easy you don't even need full party to continue.) Now if you saw JP player do that to you, you know where they got that.
What I hated about JP players through NA players point of view + when I was growing up in Japan as Half Japanese/American and looks of American. Some JP asses can view NA players/people as low class and talk shit to them behind their backs, some in regular chat thinking NA's can't understand it... too bad they had me in the party... some of them use the JP ONRY (only) to keep them away from NA's and not wanting to even communicate. (Some really do want to communicate but have hard time so those sometimes put JP Onry also when doing special events and stuff so they can better communicate, NA on the other hand welcomes JP from what I noticed and tries to communicate to make it work; I dunno some NA thinks JP players play better... I've seen really really bad JP players myself just like NA players can be elite or total crap.
Now the good JP and NA players which I played with and running my own NA/JP linkshell for 6+ years in FFXI they really love to communicate, learn bits and pieces of each others language so they don't have to rely on the auto-translate or try to be able to explain it better (for some reason they always want to learn the foul language first...lol) But we had FUN we did have some times where players had miss communication but those didn't stick too long, either got resolved or one or both just left the LS (most of the time they just came back...lol)
Their was also cultural differences so some looting methods and point system kind of confuses each other but that part they just need to learn and get used to how JP or NA cultures work to understand.
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Also adding this JP players are at least 16+ Middle School, to majority of College students or working adults, while NA players some times range from 12 yr old+ maybe younger?? I remember at FFXI Fan Fest seeing whole family with dad/mom/2 sons and the kids were young and they said they all played... I was like Whaaaaaaaaaaat!?!?!... My own family going to end up like that soon... lol so anyways yeah, age difference is also there. But guessing most in overall is 20+ in any region mainly.