There has long been a big deal about what art actually is, but my personal view is that the greatest art is acclaimed by both the critics and the common man. Art can still be good when it is recognized by one or the other, but my measure is such that it requires both. Pixar hits it out of the ball park each time, more and more lately.
Up is, in a nutshell, about a man who as a boy meets his childhood sweetheart on a common love of adventure. They share a bond that lasts through their lives, with a promise of adventure that goes unfulfilled. Finally, desperation forces the man to act on that adventure and attempt to make their dream a reality.
The primary cast of characters includes a small wilderness scout, the old man, a 'talking' dog, and a bird that is part chicken, part ostritch and part peacock. An interesting ensemble, but Pixar makes it work. There are lots of lessons in the movie, but they are very subtle. The lesson I liked the most was that you are never too old to start a new adventure. The lesson I liked the second most was that too much material possessions will tie you down.
The thing is, it appears that they started with a simple idea, "wouldn't it be cool to make a movie about a flying house?" and then expanded it from there. There are no loose ends in the story. In fact, I must admire them as story crafters, in particular the way they used the 'virtually silent beginning' of the movie in which his wife becomes the unseen character who is there through out the rest of the film because she does the talking when they are children and he does not.
I highly recommend this movie.