There are many reasons to dislike One Piece and before the Oda fanboys get their panties in a bunch I'll explain why. I used to like One Piece for one reason only. There are many reasons to dislike it including mediocre artwork and lack of depth in the characters, as well as the fact that the main character has almost no admirable qualities that translate well into reality. He's incredibly stupid which automatically mitigates his value as a human and only survives mainly because of the authors grace (Plot protection). His function is mainly as a gag character and his story lacks all emotional depth. He has no character or motivation beyond saying the wrong thing at the wrong time in a comedic sense. This was appropriate in the early chapters of the manga when the story wasn't serious but it stands in stark contrast with the later chapters. As the author tries to introduce mature 3 dimensional concepts into the story Luffy stays as a one dimensional gag character. Throughout the story the work is second rate either because it isn't a serious work or because Luffy's nature contradicts the serious nature of the later works.
There is one reason that I would have rated this above a 7 and that is because of Luffy's best quality, the reason that I rate him above Naruto and Ichigo in terms of manliness and worth and that is the fact that Luffy perserveres no matter what. This is shown in several instances, first when Luffy is rescuing Nico Robin from Enies Lobby and they show Nico Robins backstory. Then they say that her enemy is the world and that if they want to save her they will make an enemy of 170 nations and an overwhelming military force. The scenario is set up so that the reader knows the magnitude of the obstacle and one would expect Luffy at least pause before he decides to make an enemy of them but he doesn't hesitate in declaring war on the world government.
This also happens when he is trying to rescue ace. They reach level 6 ace is gone and is already on his way to the gallows protected by the largest military force ever assembled. Luffy kills himself just getting to level 6 and his companions tell him to give up and leave the rescue up to whitebeard. Again Luffy does not hesitate and says that if ace is going to marine headquarters that that is where he will go.
Perserverance is the one reason that Luffy was above Naruto and Ichigo in terms of character worth. The fact that he always moves forward in spite of the obstacles is his greatest attribute and I consider his determination in the face of overwhelming obstacles to be the high point in the story. This is ruined however when he gives up after his brother dies. He goes on a rampage destroying rocks which is understandable but the way the author executes it, Luffy seems to be throwing a childish temper tantrum. Successful people don't wallow in their own filth, they pick themselves up and clean themselves off and persevere no matter what the obstacle is. Instead Luffy encounters this obstacle as a teenager, calling himself weak and giving up on his dream, only picking himself up when he realizes that he still has his crew.
Also Luffy like I said before is a gag character. He says things that are funny and nothing more and beyond that he has no value. My ex girlfriend was from an Eastern European country and experienced true oppression as a child. Overcoming these traumatic experiences as a child gave her a strength that I could never hope to match. In comparison Luffy is a child who according to the author has strong willpower and yet interacts with his world in a childish manner.
Also the authors attempts to introduce adult issues into the One Piece universe such as racism are monumentally stupid. I don't want to offer another lengthy explanation as to why they are but even though nobles are corrupt they don't show the real reason for why the nobles traditionally oppress the weak which is to monopolize and consolidate wealth and power.
Also One Piece continues to get worse and the author continues to contradict himself. One blunder occurred when the author tried to keep romance out of his story by having Luffy reject Hancock. However this scenario is something like a mathematical proof in the sense that Luffy encounters the most beautiful woman on earth, Luffy rejects her. Luffy must be either gay or oblivious to reject the most beautiful woman on the planet and by making Luffy say "I won't marry you" the author establishes the fact that Luffy understands the significance of a relationship and still is not attracted to her. RESULT: Luffy is gay. I'm stunned that the author didn't understand the significance of what he was doing.
The author tries to move from two dimensions into 3 while withholding specific elements of a realistic story and the result is blunders like the one mentioned above. For example the author espouses the concept of fate as a benevolent force that will move Luffy from an ant thrown into a storm to a force that will shake the heavens themselves. However if fate is all powerful and benevolent then what higher force is responsible for the corruption that is present in the one piece world. It doesn't make any sense.
Also the author continues to take breaks and produce mediocre work. It goes from being a one dimensional storyline to the war between whitebeard and the world government (the high point) to being one dimensional again afterwards. The story is inconsistent in its quality and has recently gotten worse as the author continues to take breaks and set up generic plot structures i.e. having one antagonist for each character, having the characters overcome the antagonist, end of saga.
There are some things to be said for one piece. To retain the audience for so long he needs to know how to squeeze as much content as possible into a chapter and Oda does this very well but the quality of his work is inconstant. Hajime no Ippo and Berserk get consistently better but One Piece oscillates between not very good and somewhat decent too often to be considered anything but mediocre. Its saving grace is the fact that its competition also is not very good.
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