I 愛 黒執事 but for the sake of opinion curiosity, what makes 黒執事 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 もっと見る 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 黒執事 has a lot of shippeable pairings, やおい 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.