Here is the AICN interview that Dennis refers to, where a physicist/astronomer interviews Cameron... Here are some quotes:
Re: A more Alien-looking Pandora?
CAMERON: Well we started off like that... we went really extreme... purple skies, different colored plants, but we found that you just didn't buy it.... not for two hours. So we dialed it back, and put more of the exotic stuff into the nighttime stuff, the bioluminescence. But there's an argument to be made that chlorophyll might be out there... that maybe this experiment has been done many times on earth, and chlorophyll is the best way to extract energy from sunlight.
...
Re: Traveling to a nearby star
CAMERON: There's also an argument that any civilization you approach might destroy you before you get there. If you come shooting in at 0.75c [3/4 the speed of light], with a spaceship the size that we see in movies -- if you hit the planet, you'd destroy it. They don't know you can stop that thing. Are they going to take the risk?
...
Re: Floating mountains
CAMERON: Well... the floating mountains. A physicist friend of mine calculated that for a pure superconducting mountain, to break it off and float it would require a magnetic field so strong that it would rip the hemoglobin out of your blood...