10. Down: The Acting Is Uneven
It is important to note that the general acting caliber for One Piece is high, as it's a very well-cast project. That said, there is a lack of balance that will be distracting to viewers who prefer a standard tone.
If anything, One Piece's acting feels like it is one of two halves. On one side, you've got people playing the roles up and going overboard to match the goofy tone. This includes folk like Iñaki Godoy as the protagonist Monkey D. Luffy, and Jeff Ward as Buggy the Clown. These actors are having a whale of a time, and it shows, as their scenery-chewing antics are a blast to watch.
However, on the other side, you've got those who opt for subtle performances. Actors like Mackenyu as Roronoa Zoro and Emily Rudd as Nami fit this bill, as they'd feel perfectly at home in a young adult series.
This is not a criticism of the actors, as both approaches are engaging. But, there are times when the clash between subtle and over-the-top performances is noticeable and leads to dissonance.