1. The homage to the silent film era. Much of the film has little dialogue, forcing comical sequences that are all physical comedy. Not only is this a treat to watch, it must have been a lot of fun for the animators.
2. The clever jab at our consumerist culture. Without spoiling the real surprise, there's no question those piles of garbage left on Earth are signs we did something stupid over and over again!
3. There's a real love story. As with all Pixar films, the movie has heart and clever jokes. A powerful story told with care and skill.