OK I've had a nights sleep, and am back to give more detail concerning what I liked/did not like.
First of all I'd like to say that I've been reading other Gaffers impressions, and I'm very jealous

Everyone is brimming with excitement, most seem blown away by what they saw, and I really want to feel that way too. Going into the film I was ready to be wowed by Cameron, shown something that I would never forget, but even now I'm having trouble recalling anything that I think will leave an imprint on my mind for the foreseeable future.
Rather than go straight into the negatives I need to explain what I liked since my impressions have been extremely negative so far.
To start with, not enough praise can be given to the men and women that painstakingly produced the CGI for this movie. It is without a doubt the best computer imagery I have ever seen, leaps and bounds ahead of the pack. Prior to my screening I thought the Na'vi looked goofy and peculiar, and I was really worried that I would find it severely off putting during the movie, but their eyes/faces/body movement are so expressive, and so uncannily human, that you quickly forget that you're looking at a 10ft tall lanky humanoid wild cat with blue skin. Yet it's not just the Na'vi that look frighteningly realistic, absolutely everything on Pandora looks like it has been captured for some wildlife/nature show. The lush jungles, luminous plant life, and strange alien creatures all appear tangible.
Pandora is without a doubt the star of the show. It is maybe a back handed compliment to suggest that I would have been happier simply having a narrator guide us around the planet documenting the wildlife for the entire duration of the movie (fuck the conventional and tedious storyline - I really don't care). The fact that James Cameron alone designed this world is an outstanding achievement, and I was so impressed that I will probably purchase the 'Art of Avatar', or 'Field Guide to Pandora', at some point to read more about his though processes while creating his vision.
This was only the second time that I have seen a 3D picture, and funnily enough my first time was way back in 2003 when I saw James Cameron's 'Ghosts of the Abyss in IMAX. As a result I don't have much to offer by way of comparison, however I will say that other than the initial 10 or 15 minutes, and maybe the odd shot where something suddenly pops out on screen, I barely noticed the 3D. This is, in my opinion, a huge compliment because rather than try and induce a sense of awe with gimmicky tricks, the 3D is used to add subtle depth and perspective to every shot, and it truly adds to the immersion. I'm certain that I would not have been so impressed by the sights of Pandora and the authenticity of the Na'vi had I not seen the film in 3D. I will however mention that at the start of the film I found it rather jarring. Initially I thought it was my eyes adjusting, but I found that I had trouble with pretty much every scene that predominantly featured live actors in conventional human settings. While the action was taking place on Pandora, and focussed on the Na'vi, I was delighted with the 3D; however, when it switched back to capturing the humans I found it disconcerting (maybe this was Cameron's intention?)
Finally, Cameron deserves a huge amount of praise for his direction. Most of the actors give admirable performances (particularly Lang and Saldana), despite the unimaginative script and lack of characterisation. Once again Cameron proves that he is the king of action. All of the battle/action sequences are shot beautifully, with none of the hyperactive, quick-cut nonsense favoured by Bay. Unlike Snyder, Cameron makes subtle and appropriate use of slow motion as well, and overall the film is a visual treat. Cameron is still a master film maker, and if anyone is wondering whether he still posses skill as a director, the answer is a firm yes.
So why was I disappointed?
When I watch a movie I hope to be told an engaging and captivating story. I want to connect with the characters on screen and follow them on their journey. This did not happen for me during Avatar.
Funnily enough it was not ropey cgi that got in the way (I already said the Na'vi looked outstanding), and it was not shoddy acting either, I just didn't care at all for any of the characters on screen. The storyline and the characters are just so conventional, so banal, that I just couldn't truly connect with what I was seeing. I really did not care what happened to any of the protagonists. The storyline is so formulaic, and the characters so laughably predictable, that in many ways I actually felt like I'd seen the film several times before. I got the same feeling watching Avatar as I had watching a film where I had already read the entire script before hand, or read a substantial amount of spoilers; but other than a few spoiler free reviews, and the two trailers that were released, I had gone into the film completely fresh.
There are other minor criticisms as well, but none are anywhere near as substantial as the above. Yes the dialog is a little ropey, and I did expect this; however, outside of one or two scenes, and one line (that is unfortunately repeated several times), it is bearable. Horner also let me down tremendously with his highly forgettable score, although I'm not going to be too quick to condemn that since it is possible that it may grown on me over time.
The bottom line is that if Cameron had injected even half of the imagination and vision that he displayed in creating Pandora into the actual storyline and characters of the film, we would have been left with a true masterpiece, a film that I would have remembered forever. As it stands, the film just feels hollow. It's such a shame because the film excels so far in the other areas that I already mentioned, but for me storyline and character are by FAR the most important aspect of a film. They are so pivotal to my enjoyment that if they are lacking my entire experience is diminished, and in this sense the boring storyline and poor characterisation negatively impact the good that Cameron achieves with his film. I hate to draw the comparison with the Star Wars prequels, and I should stress that Avatar is far better than that, but if only Cameron had designed the worl and directed the film, but had somebody else write the goddamn script!
If I were to apply a numerival score it's be 3 stars (out if 5)