I will always feel that Kubo's first and most major mistake is that he made Aizen too holyWTF! strong and broken. He was too perfect. There should never be another Big Bad who attains the same level of power that he did. Kubo must adhere to this.
His second mistake was that the HM Arc was far, far, far too long. From the time Ichigo and co. entered HM until the time Aizen was defeated, the plot was at a virtual standstill. There was too much fighting and not enough plot-relevant info being tossed around to balance it all out, so it was tough to sit through the "smallfry fights" until the main events (i.e., Aizen proceeding to curbstomp everyone).
His third mistake: The Fullbring Arc. We knew what it was the moment they were introduced: an arc designed specifically to give Ichigo his powers back. The problem was that was literally all there was to it. Once that was taken care of, they (at the time) were killed off or were otherwise never to be seen again. So what was the point of introducing Fullbringer abilities? Nothing, that's what. Fullbringer abilities as a whole will likely not have anymore relevance.
Now, the way he's making Bleach interesting is that he's focusing on or starting to bring attention to plot points he brought up long, long ago. Stuff like Ichigo's mom, the Soul King, Zero Squad, Ichigo's origins, Zaraki's history, and the greatest point of them all, Unohana (biased). Giving us readers stuff that's actually has some effect on the plot makes the manga feel like it has substance, and that substance is what makes most manga good. If he can execute this way all the way up to the end, I'm sure everyone will conveniently forget that they ever entertained thoughts of dropping the manga.
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