Refuses to Elaborate Further - Nobody Remembers a Line of Dialogue

The only line I remember is when the military guy said "You're not in Kansas anymore." Considering that is a reference to a movie that was 70 years old at the time (and over 200 years old in-universe), that's saying something about Avatar's memorability.

Not a line, but a word. "Unobtanium."

Really, that's all you need to know to realize how god-awful and uninspired the writing is for the movie.