I think many of us are wary because all the praise seems to be pointed at the technological prowess of the film. I believe that, but my concern lies with the story. I remember reading a while ago (don't ask for source, I honestly can't remember where and it may have been here) that Avatar was supposed to be the first new movie in a while that goes to create its own universe, similar to the way Star Wars created a popular setting. If Cameron went to the length to create his own universe, I hope that he could spin a better yarn than "Dances with Smurfs", as South Park succinctly puts it.
Maybe the story is unique and not rehashing a story that we've heard several times before. However, considering the considerable amount of money that has been spent on the film, it wouldn't surprise me if Cameron decided to go with a safe and unoriginal story. Probably because spending the money on a (supposedly) incredible visual experience is a safer and easier move than creating an original story that could very well blow up in his face and turn this into a financial failure.