Something that i really like about one piece is all the different dynamics inside the straw hats. You can tell that Oda tries to make sure they are all friends, because even though you have small groups like the Monster Trio or the Cowards Trio, you always get interactions with any combination of characters. This is something that other mangakas cant do well, like im thinking of how Naruto is supposedly friends with all the Konoha 12 but he really only hangs out with a couple of them once in a while, and how there was this big emotional scene in MHA of all the kids coming to bring back Deku but it really felt empty because Deku is close with only like 3 people. Meanwhile you have all this cute and diverse relationships in One Piece and it feels very genuine.
YESSS!!!! the smallest details of characterization are the ones that make a proper fleshed out person instead of a cartoonish personality with only three features. Zoro is “stoic badass sword-nerd”, but he’s also stupid and hella perceptive of the things he cares about, and has a very clear (but very personal) way of understanding the world, and has no patience and also has a lot of patience, depending on who you are and why you’re talking to him. Robin is “classy intelectual assassin” and also likes cute things, and catastrophizing, and she’s distrustful but trusts her friends without question, and she’s at the same time a voice of reason and an unapologetic enabler of Bad Shenanigans. And those were the ones I could think off the top of my head, but all of them are the same, and the things that make them the more interesting are often shown in the itty-bitty details. Not in the big sweeping all encompassing declarations of intent and loyalty and friendship (epic as they might be) but in the small gestures of every-day camaraderie
