Before you continue down this list, this page contains some major spoilers for the new fans of the series. If you have not at least reached the time-skip part of the anime, turn back or proceed with caution.
1. Whitebeard’s death
Whitebeard was significantly injured during his fight with Akainu at Marineford. However, his injury was far more severe in the manga. Akainu managed to destroy nearly half of Whitebeard’s face while in the anime, only his moustache was slashed.
Either way, both anime and manga showcased just how much of a powerhouse Whitebeard was. He suffered 267 sword wounds, 46 cannon blasts, and shot by roughly 152 bullets. All this and he never suffered a wound on his back because he never ran away. He died while standing up but not before leaving the world with the most powerful and important last words in One Piece.
One Piece does existWhitebeard
2. Zeff’s Sacrifice
Zeff was the adopted father of Sanji during his time at Baratie. When Sanji falls into the sea, Zeff goes to save him from drowning.
In the anime version, Zeff’s legs get clamped underwater between rubble and debris so he had to cut them off in order to save Sanji.
In the manga however, it is much more heart wrenching as Zeff gave all of his food away to Sanji and ate his own leg in order to survive. This also makes a lot more sense than him surviving for roughly a month without eating absolutely nothing.
This is one of many instances in One Piece which showcases that you don’t have to be blood related to be family. Zeff was more of a father to Sanji than Judge ever was.
3. Luffy vs Doflamingo
When Luffy transforms into gear four during the final battle at Dressrosa, in the anime, Doflamingo manages to hold his own for a while before Luffy inevitably beats him.
However, in the manga when Luffy transforms, he completely obliterates Doflamingo. It was a complete one-sided victory for Luffy.
Either way, Doflamingo got the beating he deserved.
4. Luffy vs Kaido
During the very first fight between Luffy and Kaido, the anime shows them both exchanging multiple blows.
In the manga, Kaido tanks every single blow from Luffy. However, he gets up having seemingly taken little to zero damage and proceeds to use the ability shown above to one shot Luffy. They never exchange blows and you can barely see any reaction of struggle from Kaido. It was totally unexpected and delivered on what I expected from a Yonko of the New World.
5. Start of the story
There are some various difference here in how the story starts for Luffy.
Shanks appears in chapter one of the manga while he only appears in episode three of the anime.
You can see Nami sneaking around in the ship Alvida invades but she was never there in the manga. In fact, the initial start of the story is on dry land, not on the sea. Therefore, in the manga Zoro is the first crewmate to appear in the series, not Nami.
6. Hordy and Zoro
Hody never “fights” with Zoro. Zoro simply enters the water and one-shots Hody without a moment of hesitation.
There are multiple instances of this happening during the Fishman Island arc. This was the warm up arc that was supposed to showcase the Straw Hat’s progress, so seeing them even slightly struggle was annoying to see in the anime.
7. Law gives Kid the middle finger
This is simply a censorship issue that doesn’t really make or break the story in any significant way.
8. Batolomeo cuts off a hater’s tongue
When a colosseum fighter bad mouths Luffy, Bartolomeo gets extremely angry and cuts the fighter’s tongue right off.
Removing this from the anime made Bartolomeo less menacing than how he felt in the manga.
I think this is a given. Everything from manga is canon while anime contains far too many fillers, especially during the earlier volumes. I will note that everyone unanimously agrees that the G-8 filler arc should be canon because it’s extremely funny and unique. However this is only one good arc out of the countless unbearable filler arcs in One Piece.
10. Making Character look weaker than they actually are
This actually build on some of the points I made previously. For example, extending a fight scene for longer than intended such as Kaido vs Luffy or Luffy vs Doflamingo can make the character seem weaker than they actually are.
There is a significant difference between Luffy being able to exchange a few blows with one of the Yokou and getting one-shot by them. The anime also made gear fourth seem weaker than it actually is when Doflamingo was able to exchange a few blows rather than letting him get completely destroyed.
I can understand it makes for better viewing experience for casual fans, but it is very frustrating for long time fans who are heavily invested in the series.
Some changes are understandable such as Law giving the middle finger to people since anime fans tends to be watched by a younger audience so censorship is definitely necessary.