The theme “motley crew of misfits thrown together, eventually become a family” is a staple of childrens’ movies. It can be done very poorly, especially when it’s the overtly-discussed main point of the movie (e.g. Ice Age). However, a similar thing often happens incidentally to a really great adventure (e.g. The Hobbit). In Up, this theme is strong, but it’s natural part of an exciting plot that has a lot more to offer.
The first ten minutes of the movie make the protagonist a real person, with a history, that you care about. The rest of the film is adventure – with lots of laugh-out-loud moments, but lots and lots of real losses as well, including precious parts of the main character’s history. Loss, in fact, is a major theme of this film.
No plot detail is wasted. Sidekicks are not just there to be cute … they play a role. So does every detail in the setup, right down to the tennis balls on the feet of an old man’s walker. Also, look for visual spoofs of Star Wars.