Well, i was happily surprised.
No, it didn't reinvent the wheel, so to say.
But yes, it was a hell of a (good) movie. Sure it took the easy way out and mostly slapped some new-story makeup latex on the old-story actor body, but it did a good job at it. Even though it's sort of cheating, whenever a movie succeeds to stroke the deeper emotions hardwired into a human mind, it's succeeded in a universal way in being a good movie... If you didn't get it, this one did.
Not many movies do, the ones I can think of now are Braveheart, Das Boot, LotR (only on the first few watches though, it fades), and maybe Titanic (I need a re-watch, saw it last when I was less than 15). That should give an idea on which earlier works I think are parallel to this one.
As for it having an old story, it's not that unusual about movies really, now, is it? Or are you saying LotR was shit because it used the classic "a weapon that's too dangerous for anyone to use has to be destroyed" story? I seriously doubt that was coined by even the book, even being as old as it is. Or, say, Inglorious Basterds, I'm not going to believe all of you who bash Avatar for an old story didn't like that one, with an even older story I'm sure: simply blind retribution.
Now you're going to say that the examples I gave you aren't only their core plot. But then again, neither is Avatar. It has depth, just like LotR, and ballsy humor like Inglorious Basterds. Even if you're gonna say all the characteristics of said elements of depth are clichés, look at, say, hobbits. Forget that they were the first little people, or that their universe was the first fantasy universe. That, while not being a small feat, still doesn't change the fact that they're the cliché of an easy-living rural people, that, believe it or not, did already exist when Tolkien wrote his marvel.
Also, if you're wondering, i have watched both the movies, and read the book, in the original English, so I'm not just talking out of my ass.
All in all, it's not going to change the world, but it is an excellent movie, and it seems to be getting bashed on unfair grounds and way too much.