1. There is nothing original in the story or characters.
2. Those viewers who are sensitive to such things will recognize the story as a European-ancestored race-oriented fantasy where the heroic white guy saves the endangered indigenous people.
3. The visuals in 3D are mind-blowingly, earth-shakingly, damn-near-life-changingly AWESOME, and if you're going to see this film, see it in 3D, or you're not getting anywhere near the full experience.
Okay, if you want to read more about these points (mild spoilers, about the level of the previews, and not much will be a surprise if you've seen a science-fiction movie before):
1. I suspect James Cameron spent all his creativity coming up with the 3D camera system, so for the plot, characters and dialog, his brain merely assembled a script from bits of other stories and film. I'm certainly not saying that he's the only one to do that. There are so many stories out there, it's hard to come up with anything that is truly original. But if you've read Harry Harrison's works, for example, you'll be saying "Wow, these plot elements are right out of "Deathworld!'" (I suspect that anyone who has read significant amounts of science fiction and fantasy could set up a great bingo game (except while you're watching "Avatar," you won't want to be distracted by filling out your bingo card. Perhaps in later viewings).) The reuse of story elements is not a reason to avoid this film. Just be aware of it, so if you're raving about this movie to your friends who read a lot of genre fiction, don't go on about how brilliant and creative the story and characters are, or they'll give you that look usually reserved for people whose only knowledge of science fiction is "Star Wars." Just so you know.
2. Race is an emotionally-charged topic, and yet I can't honestly talk about this movie without bringing it up. "Avatar" has more themes than just race, like responsible environmentalism and personal integrity. If you just want to see this movie as a happy story of a brave hero saving indigenous people in an allegory about the importance of being connected to all other life, skip down to the '*****'. The thing that disappoints me about this film is that Cameron, in a story with racial issues at its core, could have said something even a little different from what has been told before, yet he chose to remake "Fern Gully" in space.
I probably wouldn't have thought twice about this if I haven't been having long conversations during the past several years with sanguinity, who has been doing a lot of reading and thinking about racial issues. While it's good that our popular stories are saying "Hey, we probably shouldn't horn in on others' property, slaughter them and steal their resources!", we're still telling those stories from the perspective of a white guy who saves the primitives. What if we told this story strictly from the perspective of the Pandorans? Where the hero isn't a white human, but a blue Cat-person? I don't buy the argument that audiences won't relate to the protagonist that way. Good genre authors can draw the audience in, and make the characters and story work. That would at least be a step away from retelling the same guilt-assuaging fantasy.
What's your reaction when you read this? Are you thinking about why I'm wrong? About how Cameron should be allowed to tell the story he wants? About how it shouldn’t/doesn't matter? If so, are you white? I'd suggest that all this does matter, especially to people who aren't of European ancestry.
This post says all this much better than I can. You're welcome to disagree, but I'd invite you to not dismiss what it's saying out of hand.
3. Okay, we're all back! And now I get to praise the hell out of this film. Visually, it's stunning. You know all those beautiful fantasy art paintings, with iridescent dragons and exotic aliens and impossible landscapes? That's what this ENTIRE MOVIE looks like! But instead of being static background glass paintings, it's a living, dynamic environment inhabited by amazingly real creatures. I'm sure it's still beautiful in 2D, but the 3D effects are so well done that it's like you're living in a fantasy painting (or in the case of night scenes, living in a fantasy black-light poster). The characters play out the story in a lush tropical forest, which is entirely digital and impossibly beautiful. In the Future there will probably be artificial environments that look even more real, but these are executed so well, it's hard to imagine how.
The creature effects are tremendous. The tall blue Cat-People are computer generated using motion-capture to emulate the tiny nuances of the performers' faces. Not only do the digital characters look like the actors portraying them, but they skip over the Uncanny Valley to look like living, appealing characters. I had no problem seeing them as actual beings in the environments, even when they're acting side-by-side with live-action humans. The technology has come so very far!
I was worried that I'd get queasy from watching a feature-length 3D movie (I'm sensitive to visual stress; I have to watch monkey-cam films like "Blair Witch" and "Cloverfield" in short stints so I don't hurl.) But this is so smoothly done that I had no problems the entire time. Did I mention that it's gorgeous? For a long time, 3D in cinema was mostly a gimmick for B movies, because it wasn't that good. Recent movies like "Coraline" and now "Avatar" have shown that it can be a means to improve the movie-watching experience. It's the difference between watching a moving painting of ectomorphic blue Cat-People riding iridescent flying dragons, and feeling like you're really seeing it happen.
I generally try to not rave about movies with a "You gotta see this!", but even with its faults, "Avatar" is a quantum shift in movie making, and why wouldn't you want to witness that? See it in 3D on a big screen, and I hope you're as amazed as I was.