A few short months after having broken all box office records, Avatar seems to have not really penetrated the popular culture. Maybe it's in temporary remission because it got so much publicity during its run. And maybe our collective ADD (caused by media oversaturation) has allowed it -- along with so much else -- to disappear down the memory hole.
After the original Godfather, people joked (lamely) about offers that couldn't be refused for a full year afterward. After Star Wars, "May the Force be with you" entered the nerd lexicon. After Titanic, Leonardo DiCaprio became a teen idol for at least a couple years. After Pretty Woman, Julia Roberts became a huge star. But Avatar hasn't made a star out of anybody. Zoe Saldana's stock has risen a bit, but it would have been hard for her to become a megastar since her actual face was never shown in the movie. And what have we heard of Sam Worthington since? Nothing.
Perhaps more to the point, you never hear anybody quote the movie's mostly forgettable dialogue.
You'd think somebody might at least make reference to the movie's star attraction, Pandora. But nobody does (other than me, in the previous post).