Story: The stroy is basically about human emotions, about revenge, sadness, anger, pity, hurt. For the first half, the episodes are fairly episodic, then the Shibata's make the scene. From then on, it's reason versus emotion, and emotion always wins out in the end. The anime shows that nothing is as black and white as it seems, but that human emotions cloud our judgment. It's a poignant message that often gets lost in the dynamics of the show.
Animation: The animation is quite spectacular. Between the sweeping scenery and the interesting cast, the animators had their work cut our for them. While some parts are used repeatedly, such as the flowers blooming from Ai's kimono, the beautiful art work speaks for itself.
Sound: The sound brings a melancholic tone to the anime. Considering the themes, that isn't surprising. Sakasama no Chō, the opening song, seems to be custom made for Ai Enma, while the ending song reflects a bit of sadness on the anime. The interludes, such as the dramatic music played during a vengeful death, set a tone well suited for such an anime.
Characters: Much of the cast is made of the contracts with Ai. You have the contractor, often a young person, and the subject of the contract, their tormentor. These change every episode and display various human emotional states. Ai Enma is given a dynamic and interesting persona, as are the Shiabatas, whie Ai's helpers are rather flat when it comes to character formatting.
Overall: This anime is similar to others I have seen regarding people getting their "just deserts", but it goes beyond that to show that often, revenge is unfounded and petty. The artwork is engaging, even when some of the characters are not, and the music brings the anime to life.