It was a little more serious than other Pixar movies (first 10 minutes of Up excepted), as the plot is actually about an emotional relationship, with action being kind of incidental to / growing out of that, as opposed to the usual mostly-action with some incidental emotional development. Right at the end, there's a giant emotional climax, followed by comic-relief-denouement-with-song-montag
I have some statistical complaints about gender in kids' movies, but really this one didn't do anything wrong so I will set those aside for the duration of this post. I was a bit disappointed at it for not properly addressing some of the political questions it brought up at the beginning, but I suppose it's not really meant to be The Prize in the Game and there is a limit to exactly how depressingly realistic you want your kids' movie to get about petty kingdom politics.
In the way that it initially appears to be a particular sort of growing-up story and then turns out to not be that sort of story actually, it reminds me a bit of Labyrinth. I may have to think about that comparison a little more, but there are some interesting parallels.
 ***SADNESS WARNING*** Googling quickly to see whether first 10 minutes or first 20 minutes was most accurate (I'm not sure, people seem to be using them interchangeably), I ran across a news story about a little girl with cancer who really wanted to see Up as it was coming out, but was too ill to be moved to a theater. So a Pixar employee flew down to her house with a DVD, and she got to see it about seven hours before she died. Thank you, Google, I really needed that right now. Did I mention where pregnancy hormones have been making child peril a Thing, with the crying and the weeping and the blowing of the nose?
 By Jo Walton! It's a good read.
 The Jim Henson movie. Also good.