There are, fundamentally, two different ways to approach science fiction, exemplified by Star Trek and Star Wars. The "Trek" style uses a "weird" setting to enable social commentary that would otherwise be either unpalatable or simply impossible. In the "Wars" style, the "weird" setting is the whole point: the reader/viewer can witness world-building on a grander scale than would otherwise be possible, and the plot can include elements that the audience would never otherwise swallow.

There's almost always some crossover between the two, but it's important to remember that each is very difficult to do well. Very few artists are skilled at both. A work that tries to focus on both, as Avatar does, usually fails.

Avatar is an excellent example of world-building, on possibly a bit too grand a scale for a movie: there are a handful of things that should have been explained away or lampshaded, but the explanations just wouldn't fit into an already-too-long movie. Where it falls flat is in its attempted social commentary.