Its first, and probably biggest problem, comes before we even enter the story proper. We are shown an introductory sequence of Carl's backstory, showing his interest in exploration and relationship with his deceased wife Ellie.
Telling the backstory before you tell the story is a very simple way to lay somethinmg out and can work, but the way Up does it is lazy and patronising. By showing this oh-so-sad thing that happened to the main character, complete with insipid emotional music and soft cross-dissolves, the film is basically telling us that we have to feel sympathy for him or we're a bad person. Well, frankly I'm a bad person. I'm not falling for that shit. You have to do the real legwork of spending meaningful time with a character before you can trick me into feeling sorry for them. Even Bambi is more sophisticated than this. It uses the cheapest sympathy-grabbing trick that exists - killing off the main character's mother - but it at least has the decency to do it a good way into the movie, after it's put some genuine effort into making me believe she meant something, rather than resorting to a soppy montage right at the start. If the Star Wars films had been directed like Up, the opening crawl at the beginning of A New Hope would have read something like this.
"It is a period of civil war. Rebel spaceships, striking from a hidden base, have won their first victory against the evil Galactic Empire, which is super mega evil. It's so bad, they kill children and punch old ladies. Really, that's actually what they do, they're the most horrible people and you hate them. You have to hate them, you're a good person, right? You wouldn't want to not be a good person, would you? Yeah so you'd better be really invested in these good guys right from the get go, or you'll be sorry..."
This is obviously insulting, insipid, and stupid, but it's not above Up. It's like reading about a self-insert Mary Sue character in a piece of fanfiction, who is half-elf half-human half-Saiyan, and whose parents were killed in front of her while she was raped by the bad guy, so you have to care about her now right??!!
Before it's even began, this film has managed to insult its audience.
And unfortunately this telegraphs further problems. The character arc for Carl really begins, I think, from when the bird is encountered. One could argue it's from when he meets Russell, but the first supposed "dilemma" comes when we learn that the bird has chicks and wants to get back to them, and this would require Carl to diverge from his journey.
Now, the bird, for its first few minutes on screen, was actually legitimately amusing. Every action it does in that time is clever, surprising, and funny, from its gratifying mimicry of Carl to the spontaneous way it lets Russell rock on its feet. However, for all the cleverness of these comedy bits, Up makes the grave mistake of thinking they equate to making me care about its chicks. Kevin's antics are certainly endearing, but as soon as that plot element is introduced, I basically know that this character is no longer going to be allowed to be fun. It's gone from a cartoony, inventive character to a cloying plot device, and while it's perfectly possible to have a character function as both, the transition here is jarring and depressing. It's a funny bird that makes me laugh when it goes "Waaaak!" You're not tricking me into being sad about its babies.
At this point I feel like I've been insulted twice, so any further attempts to glean my sympathy are ill-fated. The two most important things for me, the audience, to be made to care about have only induced frustration, and all of the dogs' well-observed doggy actions and mannerisms and the "exciting" action climax can't change that. As a film about a bunch of characters on a joint journey to reinvent their lives, my ability to buy into it is completely dependant on my caring about those characters, and quite frankly it fails to do that in spectacular fashion. After that, everything else is irrelevant. So the rest of the experience is spent in quiet resentment that I have to spend another hour or more watching these ugly, inconsistent character designs bounce around the screen. I understand the desire to make Russell and Carl look different, to show their contrasting outlooks on life. However, when it makes them look like they don't belong in the same film, the same world, something has gone wrong. In fact, every character in this film seems to have been designed by a different person for a different project.
Normally, if a film has good things to say about it, that equates to getting a couple of points out of ten. However, Up's plus points all serve to exacerbate or remind of its flaws. Dug is funny sometimes... but that just reminds me that Kevin was funny once too, until The Plot started and she wasn't allowed to be funny anymore. It's as well shot and cut as you'd expect of any AAA movie... but that just reminds me of the all-too-slickly-executed opening sequence, perfectly formulated to induce sadness in naive people.
For being an insulting, frustrating, bait-and-switch film that grates on my eyes at the same time it tries to give them candy, that somehow manages to be both utterly bland and jarring and unbalanced, Up deserves no more than 0/10.