I adored Avatar. I didn't find the plot particularly original, but that's not necessarily a bad thing. There's a reason cliches are so widely used; in a lot of cases, they work. I don't know, maybe it's because Avatar was actually the first film in this vein I saw, but I found it hugely enjoyable. Yes, even I knew that Jake was going to betray his own race... But it didn't bother me. I was too caught up in HOW things happened to worry about how cliched the events themselves are.
I did, however, find Avatar very original in other ways- although perhaps it's a product of my upbringing that I noticed it at all.
I was raised by parents who were in on the ground floor of second-wave feminism, and both are heavily pagan, so you'll excuse me if I found some things in Avatar very interesting. In terms of gender equality, I found it a very balanced film- far more so than almost anything I've seen, especially considering it's a Hollywood blockbuster. On the human end of things, the cast of major, sympathetic human characters is basically an even split between males and females; Jake, Norm, Trudy and Grace. The villainous main characters are both male, but to be honest that fits. That kind of mechanical callousness is usually a male trait, and it's not always a bad thing- sometimes you just have to make hard choices, and none of them are good.
On the Na'vi end of things, I took special notice how even the gender split among the warriors was, and furthermore how androgynous the Na'vi were. In terms of body build, males and females were much closer than they are traditionally portrayed. Interestingly, Neytira ended up saving Jake's life three times to his one. How many other big films can you think of where the hero goes up against the big villain first, but it's the heroine that both finishes the big bad off, and saves the male lead's life as a final touch to the battle? Heck, how many films, period, have such a thing?
As somebody raised firmly Pagan, I can see a surprisingly accurate portrayal of such a religion in it, too. The all-pervading world-goddess, her neutrality, the idea that all things are borrowed, and must eventually be given back... There are parallels here with more than just Native American cultures; there's a pretty strong Celtic theme as well, and as the old saying goes; when you steal from one source it's plagierism. Steal from five or six, it's research.
I also found the film had a succinct way of, at least partially, sidestepping the "Mighty Whitey" issue, since in this case Jake's inside knowledge of human military structure allowed him to provide the Na'vi with legitimate aid, beyond just being white. But even his whiteness is in question, since it's implied and at times outright stated that Jake is becoming more and more swept up in the Na'vi way of life; he becomes Na'vi in a far deeper sense than just dressing the same, learning the language and adopting the customs. This is, by the way, why I absolutely detest similar films such as The Last Samurai. It's surprising, actually, how many issues, both thematic and otherwise, Avatar sidesteps. It all holds together very well. Check out its JustBugsMe page on TV Tropes if you don't believe me.
For example, to those people wondering why Na'vi bows could penetrate gunship canopies the second time around; pay attention. The first time, they were shooting upwards, from a stationary position, with a bad angle. The second time around they were shooting from Ikran back, at high speed and with a perfect angle. Also, note that they always took the canopy-shots when flying straight at one of the gunships. That means the arrow has the added speed of the Ikran flying towards the gunship, and the speed of the gunship flying towards them, and yes, that does work physics-wise.
To cut a post that could go on even longer short; perhaps Avatar's plot is its weak point, however well-told a version of older stories it may be. But for me, there's far and away enough other strengths to make it a great movie.