Being someone who DIDN'T see it in 3d, I wasn't blinded by the gimmick.
The plot sucked (as you said, it's frikkin fern gully), way over-hyped, visuals were mediocre, these "imaginative creature design's" we kept getting told about turned out to be standard video game designs and in general it was a strikingly average film. If it was bad at least I would get some kind of reaction out of it, but it isn't... it simply exists.
What it reminds me of are these terribly cheesy videogame cgi films that game companies produce (FF Spirits Within, The Resident evil cg films, ect..). The only difference is those are fun because both the production company and the target audience are in on the joke that these movies are going to be kind of bad. The problem with avatar is that it's production staff, the actors, and that over-rated idiot that is James Cameron all took it seriously and legitimately thought that it was going to be the greatest film of our generation.
It turned out to be just another step in cg and 3d technology and nothing more. It got good ratings and set attendance records because it was hyped to death and for some reason people haven't caught on to the fact that James Cameron hasn't made a good film since he became obsessed with oceanography.