I have to agree, this movie is not perfect, how ever if this was made by Dreamworks, I would be astounded and recommend this to everybody I meet in the street. As it is Pixar, yes it is amazing, But not a new chapter of life, like every film they have made so far.
It would be like Jesus springing into existence and doing street magic, sawing people in half with out the use of a saw. Yeah its good compared to everybody else, but your old work was better. (PS The events depicted in that comparison are fictitious. Any similarity to Judeo-Christian Mythology is merely coincidental. )
How ever I think the real reason is that UP is 2 movies mashed together. The moment he lands The second film starts. Although the second film is good, its pulling the first down. The first film is a Journey Film about getting to the location he and his wife have dreamed about all their lives. Call it a Pixar road trip. The second is a An explorer in a strange land and a small child who tagged along by accident.
Just imagine what the first film could have been. You've seen Zathura, You know a film based mainly inside a single house is more than possible. Imagine Pixar doing Zathura. Imagine Up where the house is slightly larger, traveling through the clouds and bumping into something like, a fleet of Zeppelins. A world of people living above us in the skies. Maybe between the world wars they realized another was coming took to the skies to abandon the rest of us.
Ok this is turning into Wall-e A little bit. But My point still stands, The journey should have been the film not the running around with a house strapped to his waist.