Pixar is known for writing animation with both kids and grownups in mind. But this time, the pendulum has swung waaaaaay towards adults. The story, where different moods in a young girl's mind play the protagonists, has so many deep concepts about the human psyche, memory and the reasons behind social interactions, it almost works as a schematic animation of a university course on psychiatry. It's so original, I could not recall a similar film to compare. Obviously, Pixar has no competition out there, so it keeps competing with itself - and creates hope that Hollywood still has new ideas.
PS: The same can be said about the opening short animation before the movie, Lava. A love story between two volcanoes? That actually works? What kind of genius comes up with this stuff?