The plot may not have any originality, but the movie knows how to keep moving. You might criticize the dialogue, but it has one brilliant quality that a lot of "better" dialogue lacks - it is almost entirely free of exposition.
Think back to the movie and try to remember how many times something actually got explained to you via a lengthy scene in which the characters are talking heads narrating the situation. They're just not there - for an example, all we learn about all of human civilization comes from two throwaway lines about how Sully can't afford new legs, "not in this economy" and how there is nothing green left on Earth.
That's it.
And we don't NEED any more! With just those little snippets, we pretty much have enough to construct an entire picture of Avatar!earth. Same goes for the Unobtanium. We aren't treated to a lengthy and action-stopping list of things it's good for, who discovered it, and other boring shit - it's there, it's valuable, and that's all you need to know.
Yes, the dialogue isn't brilliant, but what I consider brilliant about it is how little of it there is.