Every type of story has its own dangerous pit-falls — things that bore the audience and are too predictable. And often times, the best stories are the ones that know how to handle these delicate story-elements in a way that is unexpected or understandable. For shonen anime, that problem is the power curve.
The power curve, for those who don't know, is a character's propensity to get stronger and stronger as the story goes on. And as the character gets stronger, so do the villains. At the same time, the hero's opposition must usually be stronger so the hero can, at the last moment, dig deep inside themselves and pull out the underdog victory. That's the underlying source of tension in almost every anime fight (and really any confrontation in any story). If an obstacle is too easy to overcome, the audience will be bored. No one wants to read a story about the time Edward Elric had to employ his mastery of alchemy to stop a purse thief. The only exception is when those mismatches are played for laughs — but that's not really what we're talking about.
While managing a story's power curve sounds simple, it's really easy to lose control. For one, you can't really go backward without pissing off an audience. Watching the hero grow in strength only to see them de-powered is super frustrating. Not only does it invalidate all the episodes you watched the hero train, but you have to go through it all over again. On the opposite end of the spectrum, making your character too strong can be just as boring. Say the hero goes from a rural child to being able to single-handedly topple a galactic empire — you're likely to lose the viewer's willingness to suspend their disbelief at that point.
While many other genres struggle with this problem, few are as susceptible as shonen anime/manga. As something generally marketed to a younger crowd (between 10 and 17), most kids are just happy to see their ass-kicking action. It doesn't matter if the character's progression is paced well. They may question it during the show's calmer moments, but when the action starts up they get distracted and forget.
Even adult fans can fall prey to the unique spirit of shonen that can obscure almost any problem. After all, when Naruto is giving a speech about how he'll defeat his enemy with the power of friendship right before delivering his Rasenshuriken attack, you can forget how over-powered the thing is (it destroys things on a fucking cellular level!) And don't even get me started on what's going on in the manga right now.
On an even more basic level, shonen stories, by nature, expose themselves to the risk of a broken power curve. Most of them are about extraordinary people doing extraordinary things, and almost everyone has some degree of supernatural power that gets stronger over time. But the difference between the stories that don't fall into the pit of power curves and those that do is the writer's ability to balance his characters. It can be tempting to make your lead character the most powerful badass that ever lived, but that's a quick way to ruin a story.
And just to drive the point home more, here are some examples of how certain stories handled their power curve — one good, one mixed, and one bad.
This story has taken great care to maintain a stable power curve through hundreds of episodes and manga chapters while including some abilities that could easily ruin everything. Everyone on the crew is capable of amazing things, but Eiichiro Oda started things off slow. Luffy, for example, was only a little stronger and more durable than most humans to start, but now he's sporting skills like gear second and haki. And while none of his abilities are anything to look down on, he is also constantly challenged by his opponents. In fact, most of his growth as a character has happened mid-battle. What's more, Oda established a deep roster of powerful opponents for Luffy and his crew from the beginning, allowing stronger foes to show up without the audience asking, "where the hell have they been this whole time?"
No one can question how great DBZ is, but that comes with the caveat of admitting how out of hand things got by the end. While characters got stronger at a good pace in the beginning, some really important characters get completely left behind/overshadowed later in the story just because they weren't saiyan. And Goku's first transformation into a super saiyan remains one of the most badass moments in all of anime, but it stopped being special when every partial saiyan could find a way to, at least, go super saiyan two. Also, by the end of Dragon Ball GT, Goku was so strong that the only person who could still train him was a reality-warping, mythical dragon.
For something that started off so good, it's almost impressive how dreadful it became. The story was mostly fine at the beginning, and most people were content to let Ichigo progress as a shinigami at a startlingly fast rate. Then things changed halfway through the Hueco Mundo arc. Suddenly Ichigo could wear his hollow mask for entire battles with no explanation, and he keeps pulling new powers out of his ass as the story demands, only to lose them after one use.
But everything goes to shit once the final battle with Aizen begins. Every time anyone thinks they have the upper hand on the magnificent bastard, it turns out they're wrong. Or they're too late and he survives by unlocking a new form. And oh look he's some kind of butterfly thing. Then Ichigo defeats Aizen with a technique that unlocks so much power it's impossible to comprehend/sense his strength. He even gets depowered afterward only to get new powers in the next arc, which is practically the cardinal sin of power curves.
But possibly the worst thing bleach did never even made it into the anime (SPOILERS BELOW). Ichigo's new fullbring power ends up becoming irrelevant once he rediscovers his shinigami powers, and he even earns a second zanpakuto. Also he's not only part Shinigami and hollow, but also part quincy. And there are even some people who are stronger than Ichigo was when he defeated Aizen with the final getsuga tensho, even though his strength was literally fucking unfathomable. Plus it's still committing new atrocities every few issues. Basically, Bleach is the perfect example of how a broken power curve can take a story and turn it to unwatchable nonsense.