I recently watched Avatar in Dolby 3D, but I also noticed the same thing in Final Destination which was in RealD.
I’ll describe it as best as I can. Say there are a number of objects in the scene. You can see that these objects are positioned in a different depth relative to the viewer, but the objects themselves have no depth or minimal depth. It is like cardboard cutouts put in different distances. For example, you see two persons and one is clearly to the front while the other is some distance behind. Yet their faces look totally flat, you can’t see the nose being closer than the cheekbones.
That’s the simpler way of doing 3D: you just layer flat things at different depths. Each stereoscopic view has the same images, just differently placed in the field of view. With more work, you can render different versions of each image for a more life-like effect (seeing more of the side on one image, and more of the front on the other, for example).
(I used to draw red/blue 3D perspective drawings in my sketchbook, and it actually worked!)
Your observations are accurate. Normally as you move your head from side to side your perspective changes slightly as does the 3D image processed in your mind. This does not happen in a movie because the camera is fixed in position and the image is projected on a flat or relatively flat plane. The perspective never changes. That’s why a 3D movie will never look truly 3D. I believe it also makes a bigger difference where you sit in the theater because you are now at the mercy of the camera perspective. Not sure how to fully explain this without drawing pictures. The camera is projecting an image as if you are in the center of the theater. When it’s in 3D that perspective is locked against a flat screen but if you are not centered then the perspective is off.
It is possible to perceive images as truly 3D (or a least a closer approximation) but it has to be done by a computer interacting with the viewer. Here is a good example of what it would look like.
All this aside, the problem with Avatar (in addition to a weak script) is that the director was working in real-time trying to deal with new technology. He was inventing/perfecting the functional technique as he was going along. IMO, he lost creative visual control trying to keep things moving forward. I would expect a much better 3D movie from him now that he’s worked out the technical aspect of it.
Personally, I found Cinerama more appealing because of the ultra wide view of the screen. By using 3 screens/projectors it was possible to add more resolution to the experience which makes wide vistas more impressive.
Even though I didn’t like the plot, I may go see Avatar again because I was in the back of the theater and feel like I was looking at a screen, versus watching a movie. The 3D effect essential made the screen more pronounced in my field of view instead of drawing me further into the picture.