This review explains with precision why I can't give The Boy and the Beast a perfect score, even though I really wanted too.
It feels like two different movies. One is magical, delightful, with a perfect mythology and sweet relationship that had me thinking it could become my all-time favorite anime. The animation is much more beautiful than it was in Summer Wars, every single beast looked interesting, and the fight scenes were very entertaining.
The other one is not exactly bad--I love shoujo--but I, unfortunately, felt extremely detached from it. It's where the movie loses the magic that entranced me in the first place, and it makes sense considering the film contrasts two different worlds, but when even the plot and characters got weaker and more superficial, I couldn't help but feel slightly disappointed.
I still loved The Boy and the Beast, though, and look forward to watching other movies by Mamoru Hosoda.