It was great world building and the new characters for the most part added a lot to the film, especially Kiri, Lo'ak and Spider. The movie really felt like it was centered on the kids, which is fine, but I also felt that because of that most of the adult characters with the exceptions of Jake and Quaritch got the shaft. I would have liked to see more of Kate Winslet's character. Neytiri should have also gotten a bit more to do. Saldana killed it with what she had but I really would have liked a bit more of a discussion between her and Spider after that whole standoff moment with Quaritch at the end. Like, does she actually hate the kid or was she just using him as a means to free Kiri? It's just weird that even with a 3 hour+ runtime there were still things like that left unexplained.
I also thought that Jake and Neytiri's oldest son got shafted in the development department. When he died I felt basically nothing except sympathy for Neytiri thanks to Saldana's powerhouse acting because we barely got to know him compared to Kiri and Lo'ak. I honestly felt more devastated at the whale death which was sad enough on its own (also thanks to the reuse of James Horner's "taking down the tree" track from the first one) but then when the Na'vi discover its body along with its poor baby? That broke my heart into a million pieces. Too far, Cameron.
But the payoff to that moment with the scumbag Captain Ahab getting his arm torn off was sooooo satisfying that the entire theater broke out into applause. Those whales...sorry, "tulkuns", really stole the show.
It definitely felt a bit open-ended considering how Quaritch flew away to fight another day and there was never a follow up with General Carmela Soprano, who I also would have liked to see more of.