There hasn't been anything like it since the original Star Wars. It's a real leap forward in cinematography, and because of this movie there is a good chance that in 5 years a lot of what will be out there will be in 3D.
If you haven't seen this movie still, see it in IMAX 3D. This is essential, 3D is not just a gimmick in this movie but is put to full use and effect.
As a story there really isn't anything original in it, and as far as visual design there isn't much original in it either, at least to people that are familiar with Cameron's influences and previous works (or anyone that's played World of Warcraft for that matter), but it's still a well-done, solidly acted and directed film.
The 3D effect in Avatar is relatively subtle but effective. The movie was actually not made for IMAX so watching in RealD or Dolby 3D may let you see more of what the filmmaker intended. Then again everyone who saw it in IMAX seems to like it so I’m sure it works.
I think the real importance of this film lies on the performances of the cg characters and advances in motion capture technology. This is the first time computer generated characters can act and move convincingly. To me that alone makes Avatar a landmark film (never mind the rather predictable storyline). There will be more and more films made with the help of motion capture and it will only get better.
By the way, James Cameron would like us to believe that with the "performance capture" software, the facial acting of the actors automatically gets transferred on to the cg characters (he said it a few times in different interviews). Unfortunately that's misleading and we are not quite there yet. Animators and artists at Weta digital (Peter Jackson's company that did Lord of the rings) still had to do a lot of work on the facial expressions after the capture.
In any case, the result is what matters. We can now have cg characters that can more or less act like real actors, that’s a big step forward.