The characters are annoying; why are we supposed to root for this upbeat twat, or have sympathy for the emo, or ignore the obvious uselessness of the stock love interest-ish?
The fillers...ok, blanket statement: if you don't want to overrun the manga, take a few months off. But don't make ~100 episodes of fillers, which only annoy people. And absolutely do not make pointless cooking fillers (cake in Bleach, some ramen malarkey in Naruto)
The story; it might be due to the fillers, but I'm pretty sure that the actual story was no longer than 50 episodes. Twat becomes ninja, twat's buddy falls to the dark side to kill his brother, twat's buddy kills his mentor and his brother, twat still looking for him. Is that about right?
But more importantly, it's in the nature of anime, or something I call Dragonball Z syndrome. Basically, you have a fairly average protagonist who must overcome some much stronger opponent. He (for the protagonist is ~99% sure to be male) eventually does after getting his arse handed to him a few times and subsequently training. But where from here? Well, how about a new, more powerful villain? Sure, so the protagonist is already stronger from the previous bout, so the new villain must be more powerful still. Keep this up for a while and you'll get characters that could probably annihilate the universe by farting, which is incredibly boring, and ludicrously contrived. Remember how quickly over 9000 became over one million?
Oh, and the whole "Believe it!" Is that also in the original japanese? If so, does it actually translate to believe it?