Bleach's strong suits became very apparent in the earliest stages of its existence; the stellar fight choreography, creative and dark designs worked around it, and tasteful themes managed a level of accessibility that was pretty impressive for its 'type' of shounen. The characters didn't show any kind of excessive depth, but really it never became anything too consequential, since at this point the immersion of the world (which, very interesting as a concept, at least at this stage) carried nicely.
As the Soul Society Arc began to open up, almost everything got an overhaul; more (and better) fight sequences, an immersive setting, and an overarching plot that held everything together so precisely it hurt a little. This was, as I see it, when the show was at it's peak, having set up these set of circumstances nicely and allowed itself the liberty to play out and focus on what it did best. No spoilers, of course, but as all the elements of this arc began to draw together, it set up one of the best finales of its time, plain and simple. It was an amazing display of long-term writing and concise story-telling, and was it not for the obvious set-up it established, I'd have been entirely satisfied with the series concluding there.
However, it kept going, and as it did the issues with the anime began to show. That focus the seasons prior showed to where exactly the series was headed was no longer apparent, showcased no better than by a blatant rip of Vegeta and Nappa's descent to Earth established in the very opening of the Arrancar arc. The events proceeding felt more like segues to the next fight sequence than they did development of a greater plot, and coincidence seemed to drive far more elements than one might like. Furthermore, as the character cast began to expand and receive more time for development, Tite Kubo's lack of depth as a character writer began to show, with more and more one-note antagonists began to pop up. The plot now felt much more tied to convention than before, which sliced into what was a real strong-suit of the more liberal, previous arc. It was at this point that I think fans began to see the writing on the walls, and while many would still follow it on the merit of still-solid fight sequences and decent atmosphere, a lot of what made the series great had been lost.
The filler arcs and the "canon" proceeding the Arrancar arc were acting extensions of the issues previously established; as the lore of the world expanded, the tone and portrayal of it all did not, leaving a gap of uncertainty that really cut into the overall immersion of the anime. Kubo has been trying to intertwine similar elements of his early successes into his proceeding arcs (significantly the ideas of in-fighting between factions), but none of them really tend to go anywhere, which is upsetting for someone who saw it done so well by that very person.
In spite of all the negatives I note with this anime, I feel strongly that it does have a lot of enjoyable qualities to it, and I'd argue that the Soul Society arc is something of a modern masterpiece. It is heavily overrated for sure, but there is an obvious appeal to aspects of it, and if you can forgive the plot contrivances and somewhat underwhelming character focus in the later bits, then Bleach can be a pretty fun watch.
