I saw it for the 3D, which was awesome. I can't wait for a sci-fi film with this technology that doesn't poke a black hole in space-time with its inescapably sucky plot.
Pandora was beautifully rendered, and I particularly enjoyed Sigourney Weaver's performance. Other than that I simply cleared my mind and tried not to get clubbed over the head with the message.
At the risk of sounding like George Lucas (five Hail Marys for even mentioning him), here are some things that would have made Avatar better:
1. Make the Na'Vi a bunch of jerks.
Watch Mel Gibson's Apocalypto and make the majority of the Na'Vi as ruthless as the Mayans. Have Neytiri be part of a small group of people who want to save the Home Tree. Let the majority be in league with the humans, trading mining rights for weapons to crush smaller tribes.
2. Make the MacGuffin (Unobtanium) not a MacGuffin.
Rather than a useful mineral that is otherwise irrelevant, why not a substance that will help heal the broken Earth? Perhaps a substance that will bring life back to fallow soil, or help scrub the air of toxins.
3. Make us care about the humans.
As in #2, give us a reason to care whether or not the humans are successful. Restoring Earth is a laudable goal. Also, create political pressure from liberals back home for a less invasive mining operation. The heavy-handed methods of the humans on Pandora should not be universally accepted. Support for the Avatar program should come from home and be obvious, like a vocal senatorial panel.
4. Make us care about Jake Scully, or at least give him an edge.
If Jake cares about restoring Earth, then he'll be conflicted about harming the Home Tree.
5. The final battle should have no clear winner.
Everybody loses. The Na'Vi are worse off after driving the humans away, and Earth is still screwed without the helpful material. The happy ending is only with Jake and Neytiri and their successful relationship, and their hope for the future on a local level.