I'm getting rather sick of reading reviews criticizing this movie on the basis of its perceived lack of originality in the storyline department. When did people stop enjoying classic plots and themes? Remember fairy-tales, folktales, sitting around campfires listening to stories you already knew but relished nevertheless? This movie is simply wonderful -- stunningly gorgeous visuals, classic characters and plotlines, archetypal themes, all skillfully and impressively executed. If you enjoy these things and don't expect every film you watch to be the absolute pinnacle of originality and complexity -- in other words, if you still have a part of you that delights in simple joys and wonder -- then you will probably adore this movie.
I also think many viewers are missing a crucial plot point, which may have been too subtle for its own good. Read on only if you don't want a -
*SPOILER*!!!!!!!!
A major crux of the story is that when Agito's body has been absorbed by the tree, but his consciousness is intact on another plane, the Forest reveals to him the truth about the relationship between itself and the humans. Agito learns that the genetic admixture that gives humans extraordinary strength and eventually turns them into trees is really a two-way exchange; it also changes trees in the Forest, causing them to give birth to new humans in giant fruits. These new humans appear to be a stable product of the combined genetic material of plant and human. The Forest, seeing that Agito can possibly teach humanity that there is no need for either hostility or separation between themselves and the trees, grants him his human form back. The point here is that even the "Neutrals" are not truly living harmoniously with the forest; they still hold themselves apart, trying to preserve that which is in an inevitable process of transformation. Hence the veiled, cautious passive-aggression between the Forest and Neutral City, whose denizens do not understand that their days are numbered and that the strange little nymph-like creatures who speak for the Forest truly represent the future of humanity.