It also helped that in DBZ a good chunk of the power creep was from random alien a-holes showing up, as opposed to people who are all related to the same little insular culture of dead-men-walkin' in one way or another.
Yeah the major difference is that Bleach had a very established universe and while Aizen is ridiculously overpowered, he is meant to be something completely unknown and new. Yet they keep introducing newer and newer threats who are in par if not more powerful then soul society captains (ignoring that whether or not Ichigo is as strong as a captain depends on what side of the bed he wakes up on) and who often break the very established rules of the universe.
It even gets better because Bleach even outright states at a few points that in general captains eventually reach a peak and really can never get past it. They even go as far as to state when certain characters reach maximum strength.
So basically it just turns each plot into: "Ohh no! It turns out there was actually an even stronger villain hiding under the carpet! Quick Ichigo, now you must also become half-maid to defeat him that way you will be half-human, half-soul reaper, half hallow, half-quincy, half-alicar, half-vampire, half-squeegee, AND Half-maid at the same time"
While in Dragonball Z... Each threat was a rather believable next step up. You had a Demon, Then the Demon in his Prime, Then you had The main character's much more powerful brother, then you had the most powerful member of the main character's race, then You had a galactic overlord, then you had a Genetically perfect being, then you had a galactic monster being spoken about by gods who run the entire universe, Then GT happens but lets pretend it didn't happen (even if I actually liked it)
But I don't know Bleach might be bad writing, but it is awesome writing.