Firstly, posted anon because I can't see this not getting flamed. However...
I really enjoyed Avatar, and although I might not class it as one of the critically best movies of my life, I might class it as one of the best cinema experiences of my life. I've always enjoyed films, on an intellectual level as well as an entertainment level, and in some ways this has spoiled things for me. Since I was a kid I've been using computers, and because of this, I witnessed the transitions through graphical techniques as it went along, and the unfortunate consequence of this was that I stopped enjoying films with computer/CGI effects in the same way because they simply do not absorb me from a graphical point of view. That doesn't mean I don't like CGI movies, I did enjoy LoTR, presumably because it was a generally decent telling of what I expected already in my imagination.
Anyway, the general point is that I remember (and this belies my young age) going to see the Back to the Future films as a young kid, or the first time I saw Star Wars and Indy, and Jurassic Park, and actually being thrilled and amazed at what I was seeing. I saved myself for Avatar, I didn't watch a 3D movie in the runup because I was waiting for something truly special, and when I went to see Avatar, for the first time in a great many years I was absorbed fully by the mixture of 3D (sorry Kermode) and quality of the CGI. This is why I forgive the story being generic (we forgive plenty of other films for the same problem) plot, it was well-told enough not to be an issue, and it is also why I forgive the (a bit above) mediocre acting, and it's not because I have a hard-on for Cameron, or because I love the Navi, it's because the film made me feel like I was a kid again, and that's a pretty amazing feeling.
Also, I didn't notice the length at all, it flew past, the missus was holding in a pee for the last hour so she did notice it. Lastly, the only thing that even annoyed me slightly was the Horner 'danger' motif. GAH