Oda plans far in advance, as well as world builds and goes into the history of his Manga. It's also very political, and it's interesting to see pirates having an influence on politics as well as the World Government allying themselves with other Pirates.
In spite of it being a battle shonen, the Straw Hats don't usually train that often and come up with abilities when put in a certain situation, then they unlock more power because of this.
As far as his character writing goes, usually there's not much development, but there's usually no internal monologues, so we can attempt to interpret a character's actions.
Two of the biggest flaws of One Piece are its repetition and troupes. If not for these two flaws, the series would be near perfect.
