-World Building
-Characters aren't ignored
-Character development
-Artwork, seriously there's barely any "white space", most if not all panels are filled up.
-SH's backstory
-Certain themes it covers (i.e. slavery, racism, etc.) Even this whole stuff about the WG trying to hide the history of the void century from the public can be related to real life to an extent. The government hides stuff from the public.
-Mysteries (i.e. Void Century, Will of D., Dragon, etc.)
-Fights & Upcoming fights
-Every arc feels unique and has it's fair share of inspirations from real life.
-Arcs have a "snow-ball" effect and it's seems to be a lot more prevalent now that we're in the NW. Not one arc felt like a "waste".
-Haki (kinda of a spoiler) adds another layer to fighting styles, same goes with the Devil fruit idea.
-Oda really gives you this sense of adventure through his drawings and story-telling.
-The comedic relief from Luffy's stupidity is always funny.
-Although the story is for the most part "light-hearted", it can be really dark at times. So it has that balance.
-Flashbacks actually have meanings to them & it's really to the point where most fans WANT flashbacks. Just shows how much the fans are invested and want to see the history of the verse.
-There is always one SH that you can really relate too. (Sanji would prob. be mine)
-DF abilities are unique/interesting
-The views of what the government sees as justice being justifiable or not.
-Seeing the story through the "criminals" (pirates) POV
The list seriously goes on from here... but I love OP it's a masterpiece.