One Piece since the beginning of the timeskip has been a good example of that.
For a while it was a flawed but phenomenal adventure that delighted partially by being unpredictable, while building dramatic momentum, the bonds of friendship, crafting a beautiful paradox of dark and goofy in its universe and making it credible enough that a five -year old minded spoiled brat would be capable of becoming the king of the high seas.
Luffy is kind of the Tom Cruise of anime characters. He is small but he seems bigger because he is capable of specatular stunts, he has a trememnous s**t eating grin which was cute (for a while) and he is a little naive. And he has a great crew. They are the kind of characters you fall in love with, but falling in love is always a superficial experience. When you stand back you realize that they are all stereotypes; the coward, the alcoholic, the womanizer, the hunk, the perverted grandpa. The only character that resembles a real human being is Robin, who is so silently compelling that she should have been the start of the series.
The crew go their separate ways for two years after about 500 episodes. Luffy hides away on an Island to train with a sensei in the art of Haki, much like Luke learns the force from yoda. Unfortunately he gains all of the action figure benefits of the training but none of the wisdom.
Right from the moment Luffy reunites with his crew, not as a commanding captain, but like a little brother who is seeing his older brothers and sisters, the ship called ONE PIECE was sinking.
The universe has gotten too big for Luffy to NOT grow up. His immature tendencies are no longer entertaining. Rather than make him a character, Oda is content to leave him as a formula: he is tacky during funny scenes and aggressive and supposedly sincere during action scenes. That sincerity feels phoney now, simply because it is not in the nature of the person that Luffy is. It is as phoney as Donald Trump's ridiculous promises to America, painting an illusion of a utopia.
A show about a candidate for a pirate king needs a strong character for a protagonist, not a sketch of a kid. Now this wouldn't be a problem if the show were purely comedy, but no it also want to be a heavy drama at times too, and Oda's ambitions have gone beyond his ability to write them well. As a drama, The tonal shifts are so inconsistent. Sometimes it is played as farce, and sometimes it is played more sincerely. More often than not though, the darker subplots involving themes like genocide are done very two dimensionally, and they tend to serve messages that are very glib and lead to backwards minded sequences. Robin (for example) is an adult from a dramatic world who becomes a disney-derivative damsel in distress at one point, and her prince charming are kids from a comedic one.
This is just one of many examples of Oda destroying the integrity of his characters by being too condescending of them. He makes the mistake of thinking that in order to make a character sympathetic he has to make them seem childlike in innocence. such is the case with characters like Luffy's brother Ace, who goes from a bad ass to a crybaby. Indeed some of Oda's philosophy is a complete insult to our intelligence, No subtly at all, and the emotional scenes are always breastfed to us.
The Devil fruit concept was cute at first, but now it's just silly, like an idea stolen from a video game. There are too many fruits, yielding too many powers and likewise, there are too many characters in the universe. Some of them, in fact most of them have become far more interesting than Luffy, and his pals at this point. Since the timeskip, the Strawhat pirates have become increasingly less compelling and intimidating, and are now a bunch of pansies in silly costumes. We don't really see an increase in their straight because the antagonists are always two steps ahead of them in strength, when they should be more like 1 step ahead. As a result the fights all feel like recycled sequences.
One Piece is a show that has gone on too long and yet disputed the distance we have travelled, it feels like we have gone nowhere.
