It was a passable timewaster, and it's nice that Cameron still knows how to direct an action scene (and build the tension up to it). But you know what really bugged me about this? I don't think the awkward dialogue, thin characters or predictable plot would have mattered nearly as much if Cameron had actually done what he'd set out to do, which was bring to life a truly alien planet, ecosystem, and culture.
Yeah, sure, it looked great, and there were some nice touches--the floating mountains were an interesting terrain, and the "neural network" among the life forms was a cool idea that really wasn't explored enough. But other than that, and the bioluminescence, the whole world felt half-assed.
I'll tell you what my big hope going in was: I was hoping to see some lifeforms behaving like real animals, alien or otherwise. None of this "stopping to roar in the hero's face instead of just biting his head off" shit (the hammerhead rhino thing gets a pass because it was making a threat display, but then seconds later you've got the tiger thing giving Jake all the time in the world to run away, wasting energy snapping and slashing at him, etc. etc. etc. That's not how hunting animals act. I wouldn't make a big deal out of it, but this is supposed to be the movie's raison d'etre, right? Immersing us in a realistic biosphere?
Then you've got the fact that, except for the dragons (which are the kind of thing we've seen before anyway), all the Pandoran creatures are just analogues of Earth creatures. You've got monkeys, birds, rhinos, tigers, dogs, bugs, horses, blah blah fucking blah. And of course this extends to the Na'vi being insultingly stereotypical native Americans. Wow, great job creating a dazzlingly original alien world, there, Jim. Jimbo. Jimmy-Jim Jim. Seriously, other than the USB ports in their heads, was there a single idea about the Na'vi culture that wasn't imported direct from Earth cultures?
District 9 has officially made a mockery of any Hollywood movie that thinks aliens have to be human-looking for us to relate to them. And Wayne Barlowe, who designed the alien life forms for Avatar (apparently he's working on the upcoming John Carter movie as well?) has a book called Expedition, detailing an alien biosphere that's about a dozen times more original and interesting and ALIEN than anything in this movie.