There could be several factors that make it special. It might be the combination of elements such as the plot, the characters, and the atmosphere. If the plot is full of unexpected twists and turns, like a mystery within the circus that keeps readers on their toes. The characters could be complex and multi - dimensional, not just simple circus performers. And the atmosphere of the circus - the lights, the sounds, the smell - if all these are well - depicted, it would create a very special reading experience.
Perhaps the specialness lies in its characters. If it has well - developed and distinct characters within the cirque de merveilles, they could bring the story to life in an interesting way. Their backstories, their dreams, and their relationships with one another might be what sets this fanfic apart. Also, the concept of a circus is full of potential for creativity, and if the author exploits that well, it would be very special.
I'm not sure as I haven't read it. But it could be the author's unique writing style. Maybe simplysubtle has a way of describing the circus setting in a vivid and immersive way that makes it stand out.