I amor kuroshitsuji but for the sake of opinion curiosity, what makes kuroshitsuji so special?

I think it's a mix between the plot, the art and the setting. And of course, because of its characters, which are round, well-written and charismatic (ones más than others). Some people like it for the plot because it's a mature and dark one even if it's listed as a shonen series. Others would see it just for the yaoi, but get over it, even if kuroshitsuji has a lot of shippeable pairings, yaoi is not its axis. Others would see the series because of its setting because Victorian Era was an important one and is not so common to find works based on it.