For me, bleach went terribly wrong after "the antagonist". The chapters just stopped being interesting, anything new that happened was average, even the unexpected parts.
There is, of course, the matter of Ichigo's character being one-sided, and his friends being just his lackeys ever since their side-stories were finished, but more than anything, it's the fact that Aizen has been the main villain for over 150 chapters now, and the good guys have done nothing but get their asses handed to them by his goons. Battles that were meant to be short turned out to last forever, and while the espada are finally narrowing down, nothing stops KT from unleashing the secret hollow corps that will kick everyone's ass and then dish out the vizards to help, and then go back to the main cast....this is so boring I can't type anymore
note: the thread title says [manga], no need for spoiler tags~