Before I begin, I just want to say "MY OPINIONS ARE SRS BIZNESS! SRSLY!" Okay, now that we've gotten that little outburst out of the way...
You've probably heard of this complaint in some form, where my first memory of it was Dragonball Z. A lot of the "plot" seemed to be summed up thusly:
Goku: Ha! I will vanquish you with my ultimate attack!
Villain Du Jour: Well guess what? I have an attack that's even bigger!
Goku: Really? Damn, I guess I should use this unknown superweapon that was never hinted at until now!
VDJ: Well, your attack is strong, but I have this other powerful attack!
Goku: Ow! That hurt! Give me more episodes to charge up my even bigger attack!
VDJ: Oh no! I am defeated!
Goku: Well, it's a good thing that he...
Other Villain: Surprise! I'm even stronger than the last guy, and everything you learned to defeat him is completely useless against me! Looks like you'll have to pull even more superpowers out of your ass!
After that implied "Fuck!" moment, the aforementioned summary would pretty much be recycled ad infinitum, though with different characters and attack names. At first, I thought this was exclusive to DBZ, but then I saw other franchises fall into this trap: Bleach seems to have turned into a contest over whose sword/ban-kai/whatever is bigger. Naruto started out interestingly, but then turned into who can summon bigger monsters, making one wonder if they even cared about the actual "stealth" aspects of ninjitsu in the first place (as an aside, the only character I can think of who actually used stealth in his attacks carried a gigantic steel paddle on his back). Inuyasha seemed kinda cool when I was younger, but looking back I can now see how Naraku's spirit would conveniently escape every time the protagonists supposedly killed him, and how each time he would send demons that required ever stronger attacks and training montages in order to defeat until I just stopped caring.
One common thing I find in each of the above animes is that there never seems to be an actual ceiling on the power one can have. It seems to be a purposeful choice from my point of view: by removing the maximum limits to the power of the characters that are in the story, or that will appear later in the story, you have an excuse to keep writing new plotlines. As long as you can keep selling these books, you can keep grabbing a paycheck, even if you've practically run out of ideas on where to take the story. There doesn't have to be forward progression, just upward progression. Kagome can spend God-knows-how-many years never actually finishing off that Shikon Jewel as long as Inuyasha has new villains to slaughter with his gigantic sword, where the only change appears to be how often he screams between each strike. If people keep buying the books, then the writers can keep bringing home the paychecks.
Maybe I'm being an elitist curmudgeon here, but I like the shows where there is a sort of ceiling to how powerful the characters can be, where the enemies aren't simply defeated by pulling new superpowers out of nowhere, but by using different strategies. For example, you can argue how Lelouch's Geass power was a sort of "deus ex machina", but at least that power had a ceiling, and at least that could be countered by villains who didn't resort to screaming even louder than him. In Cowboy Bebop, there wasn't a super-starship that could only be shot down by a ten-year-old piloting a super-mega-ultra-starship. Hell, the crew of the Bebop actually screwed up some of their missions, and couldn't fight the same adversary in a larger form either. Even the climactic battle scene was somewhat down to earth, with Spike holding a simple gun vs. Vicious with a sword.
I could list tons more examples for either side, but I think you all can see what I'm getting at. Why is it that some series just continue long past their shelf life, when it just seems the writers are recycling different plots with ever-expanding powers that were never hinted at before and will possibly be useless later on? Isn't it better to have a sort of power ceiling?