I actually also saw it in both 2D and 3D.
I didn't notice a great deal of difference between the 2 versions, except in the close-up shots where the focus issue is easier to distinguish. I think that it's a testimony to the quality of the filming and CGI that the 2D version looks as good as it does.
For those of you waiting until the DVD comes out, you better have a freaking fantastic TV and sound system in order to do it justice, or you will miss out on the scale of this movie.
As for it being a 'rip off' of other movies, yes, it has a great deal of similarity to others. I felt that it closely resembled Ferngully as well, especially in regard to the relationship between the main character, the alien girl, and the girl's betrothed. Even the mother had a 'twin' in Ferngully. Also, the scene where the dozers come to the tree (and the tree itself) are similar. The main difference is the antagonist - a jar-head in Avatar, and an evil spirit/being in Ferngully.