I would say from what I have seen of Bleach and less failure series (like TTGL, Berserk (I'm already several volumes in, it's THAT good), and... others I can't remember now) that the biggest difference is quite precisely what Vector said. Characters who never change (URGH DETERMINATION) and always get abruptly leveled up when they need it and never suffer setbacks are boring. A series, to be compelling, needs good characterization; it needs adversity overcome with more than OH HEY HOLLOW SIDE IS HERE TO BAIL YOU OUT AND IT CAN DO EVEN MORE THINGS NOW HURR HURR every time the going gets rough. TTGL is exactly as much about [REDACTED] in Episode 8 and the characters moving on from it as it is about the main story arcs. In Berserk... well... Yeah. If you've seen/read Berserk you know full well what I'm talking about. It's a series where sometimes in spite of all the efforts the protagonists make, in spite of all their determination and valor and ginormous swords, sometimes they still fail in a catastrophic, heartbreaking manner. Not fake "Urgh I'm going to need to achieve Bankai in 3 days" failures. ENORMOUS, COLOSSAL, HORRIBLE things happen to them because the bad guys are just better at what they're doing. And it's compelling. How can someone empathize with a character who abruptly gets better at things when we have to work for it, a character who never fails when sometimes we suffer defeats even if we try our best? If they can't at be humanized, why should we care about them? If we don't care about the characters, why should we watch or read a series? If we know that in the end the heroes are just going to go and get asspull bonus powers and win every time, where's the drama? There's no excitement in that unless you do it REALLY GODDAMN SKILLFULLY.
tl;dr: Vector is as correct as she is awesome :3