I think a part of the movie that turned me off was the behavior of the main character. He fell in love with an alien and then quickly betrayed all his friends, the humans. I can't be the only person who found this off-putting and unsympathetic. This is not a trait that the main character of a blockbuster should have.
This is a familiar trope in fiction. Dance with Wolves, Last Samurai, and Jon Snow/Ygritte. But in GOT Jon Snow never betrayed the Night's Watch (he was a spy) and in the Last Samurai the main character didn't betray Americans, just the Japanese that formerly employed him. The Betrayal of the Avatar guy was a bit too far IMHO.