The reason I dropped the series is because Anita stopped kicking ass and she stopped being in danger. Of course a lead protagonist is highly unlikely to die, but by OB it became clear Anita was never going to be placed in serious peril again. Anita Blake: Vampire Hunter, a series that started out as an action-horror fantasy, got boring. That's an unforgivable sin as far as I'm concerned.
One can say the secondary characters can be hurt or killed, but IMO, the secondary characters are barely entities in the series, which makes it hard to care about them. It's all Anita, all the time; there's little to no room for character development for anyone not Anita. When a TV show, movie, comic book, or book series narrows its focus to the holy importance of a single character and their whatever, then that's my jumping off point.
The plot holes and second-rate porn didn't help either.