It’s not that one reading is not enough to understand the intricacies present, but that the beauty of the work demands it. Honestly, it is an injustice if not read more than once.
Characterization, and character development. Story flow. Thematic coherence. Plot, and plot development. Paneling. Drawing; background, foreground, middle ground, and the characters. Narrative, that is how the story is told. The emotional depth, and the ambiance of the manga. Everything is simply perfection.
In terms of the use of temporality it reminds me very much of Shinjuku Lucky Hole. Another gem.
The overarching themes are Death, Life, Nature, and Consumption (the enabler of life, and the lack of which is a sing of death). Niini no Mori addresses the question of the “Other”, the duty which we have to one another, and the fundamental vitality, Life, which each living creature possess.
In many ways Niini no Mori is concerned quite concretely with a) the question of Difference, which may be physical or mental, and then what this difference then allows and disallows, and b) the Body, or to be specific the bodies which seem to embody this fundamentally unbridgeable difference, however it is not so. True, the bodies are different anatomically speaking, however the differences which the bodies are then said (or rather thought) to inherently possess is a false claim.
Indeed, the marking of the body as Other, seeing it as inferior, unrabbit/human, and thus fit for “sacrifice” and consumption, is one very concrete way through which the manga address the question of Difference.