I liked that the standard "connected to nature" crap wasn't just mystical storytelling, but they actually had natural neural links that could connect with other animals and plants. Pandora seemed like a really well-thought-out world that might be Earth plus another billion years of evolution. The floating mountains were cool, and I think it was actually plausible that it was some sort of biological interaction with whatever the "unobtanium" was that made a type of antigravity effect possible, probably moreso because they didn't try to bash you over the head with technobabble trying to explain it and just suggested it. The realism and believability of the world was one thing that definitely took this from being a mediocre movie (I don't think anyone's going to contend that the story is a straight-up rehash of things we've seen a hundred times before, and all the characters were pretty flat archetypes with very little to make them interesting) to a rather exceptional one.
The visuals, of course, are the other thing that did it. Even seeing this in 2-D on a regular film projector (you'd think years of selling 20 cents of popcorn for 10 bucks would provide enough money for upgrades every once in a while, but I guess not) was an absolutely incredible visual experience. I can't think of anything else where I've been so blown away by the effects - the battle over Coruscant at the beginning of RoTS comes close, maybe.
All in all, Cameron achieved what he set out to do: he out-Lucased Lucas. He made a movie more expensive, more CGI-heavy, and with a weaker plot than any Star Wars film
(And you know what? I loved every second of it and will be seeing it in 3-D as soon as I can get somewhere resembling civilization)