I saw this movie twice now and I still love it. People criticize the story for its apparent lack of plot. It may have just been my family, but we thought the plot was GREAT! As a writer myself, I found it engaging, the characters lovable, and all the emotional ups-and-downs throughout the story very...emotional. It may have several themes found in other stories, but how many stories have taken themes from, i.e., Shakespeare? Unique stories may be nice, but not always feasible. I felt that Cameron took themes from other great stories and used them to make his own original world.
Definitely see the movie if only for the CGI. But don't discredit the story just yet. Perhaps it's not one for everyone to enjoy, but see for yourself!