Naruto became quite poor in terms of storytelling after the timeskip. It also opened up several annoying plotholes. Stopped reading a couple years ago.
Bleach, after the Soul Society ark, just became boring and repetitive. Story and character quickly took a back seat to cool action scenes, as they basically repeated the exact same story arc. One of those mangas that had a great premise, but just became lazy in its storytelling, not really going anywhere and opening up multitude of plotholes that the author seems to have no intention of filling. Stopped reading about the same time as Naruto.
Yu Yu Hakusho was fantastic almost all the way through, right up until they started bringing even minor characters back from the dead.
Spoiler (mouse over to view)
A particularly bad example of this is when they brought back Game Master, which took all the weight out of one of the most emotionally powerful scenes in the manga. The final arc started very well, focusing on drama and character rather than fight scenes. The subtlety with which Yusuke was coming to terms with the fact that he is a demon outdoes pretty much any shounen manga in terms of the whole "demon inside me that i can't control" cliche'. But then, instead of finishing what he started with these characters, the author just ends it with a Dragon Ball-style martial arts tournament. What a shame for such a great manga.
I'm gonna get lynched for this, but much of the second half of
Fullmetal Alchemist had me underwhelmed. By the end they were more focused on the action and political struggles than on the Elric brothers, who are the heart of the story. I think the anime handled the brothers' story far better than the manga. It had so much more heart and emotion, even if it did occasionally devolve into melodrama.
Ruroni Kenshin. The final arc wasn't bad or anything, but it was certainly underwhelming when compared to the greatness that came before.
Spoiler (mouse over to view)
I think they should have shown us the final arc earlier and saved Shishio for the manga's finale. Such a fantastic villain. Enishi doesn't even compare.
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