I think a lot of it comes from the fact that there is very little that is original about Avatar (and, arguably, there is very little substance generally . . . that is to say the original was a hollow shell, bolstered by the gimmick of 3D).
Its like if I made a fantasy world called "central Earth" that included short, fur footed creatures called halflings, miserly, grumpy, bearded "gnomes," and pointed eared, wise "ancient ones," all of whom were warring over a bracelet of power and then said it is an original property.
Avatar is, in essence, Dances With Wolves/Fern Gully combined with Call Me Joe and a few other sci-fi novellas. So the story doesn't really have anything remarkable about it that lasts the test of time.
Further, the effects, the very gimmick upon which it is built, has been far surpassed. So at the end of the day, the question becomes, what is really special about this property? I see very little.