The visual detail is incredible. It is a must-see in 3D. It was designed for 3D.
The broad plot ideas are nothing special, particularly if you’ve read much sci fi or fantasy, but that’s okay. The moral theme (humans in general and corporations in particular are evil, tree hugging Gaia worship is cool) is rammed down your throat too much — I would have liked to see something a little more Alien-and-The-Abyss-meet-your-classic-dragon-fantasy-novel and a little less Princess-Mononoke-and-Pocahontas-read-the-Green-Left-Weekly — but, as my brother points out, it’s not a film written for me, but for it’s target audience, who apparantly like broad brush strokes and simplistic themes.
Even so, I would have liked some proper character development and better acting. The bad guy is cartoonish. The corporate stooge is simpering and never displays any of the internal conflict the role clearly calls for. This standard of visual detail will soon enough be the new normal and once that happens, nobody will remember Avatar, which is a shame.
Mike Russell has written a review that I generally agree with, here (lots of spoilers).