FYI, Izaya is the primary villain, not a secondary characterProbably way Ryohgo Narita wrote this one character izaya in durarara!! He just keeps taking over the show and it just seems like he's controlling the universe when he's supposed to be a secondary character and all the other characters are clueless
agreedSomething that made me drop a "good" series you say... Truly good shows don't have such things. Alas, quality is highly subjective, so I thought DRRR was amazing.
To answer your question: I have dropped very few shows personally, but if there was something that did it for me, it'd be bad writing. The symptoms are varied, shallow motives, hand wave-y explanations, awkward interactions, and the feeling of being contrived are only a few examples.