I actually do think the characters had quite a bit of depth in the second one. To me, I think people get too caught up in writing "good bad buys, bad good guys" or characters chockfull of one liners.
Avatar's characters might be archetypes, but I do feel they were well written and developed. Plus in a way that's why James Cameron is able to appeal to so many people globally, its familiar no matter your culture. As someone who thought the first one's characters were dull as rocks, I did genuinely like Jake's kids in this one and was scared for them in the end, not to mention eagerly awaiting what's next for them in part 3.
Granted yeah, I'm surprised the story structure is the exact same as the first one and wish he changed it up, but in the end this movie didn't cross 2 billion globally just on the special effects alone. At least that's my take on it.