I first saw Avatar in 2D, and it was beautiful. The visually rich environment of the planet was extraordinary. Many of the scenes seemed to be lifted (unacknowledged) from Roger Dean paintings, but I love Roger Dean paintings, so this was a joy to watch. I enjoyed studying the extensive detail filling many of the shots---the luminous forests and arcane wildlife made every "shot" (to use the term loosely) a feast for the eyes.
I decided I should take advantage of the 3D version while I could, so I went and saw it in one of the highest-quality screenings around. While there were times that the 3D effect was very cool and gave the images a more visceral quality, overall I found the effect distracting and not enjoyable. While objects in the shots did seem to exist in three-space, I found everything to have a sort of transparent flickery quality. By the end, I had a bit of a headache and my eyes felt funny.
Reflecting on the experience, I realized the origin of this feeling might be rooted in a quirk of the way the 3D effect is created. While sending a different image to each eye does create a stereoscopic focal point in space, the optical focal distance is always the actual distance to the screen. When viewing normal objects, these distances are always correlated. However, in a 3D projection like this, the stereoscopic distance and the focal distance are different, forcing the eyes to operate outside their normal parameter space. I suspect this subtle visual anomaly is behind the somewhat weird look of everything and the cause of the discomfort I felt by the end.
There was another factor I found annoying, at a more conscious level---the omnipresent awareness of the "camera". Even though the 3D image appears "real" in some sense, the frequent cuts and camera motions collide with that sense of reality. But an even greater sense of the "camera" comes from the fact that many of the scenes are rendered with a limited depth of field, as if they had been shot with an actual camera. Thus, for example, when one of the seeds of the tree of life goes floating by in the foreground, it's a big white blur. You can cross your eyes and look at it, but it's still blurry. And, there's still the problem that it's least blurry when your optical focus is aimed much further away than the apparent distance to the object.
Depth of field is a useful compositional tool, and can be used to great effect to direct the audience's attention to different parts of a shot. But I still like to study the whole frame and look at things that aren't the central subject. That to me is part of the point of seeing a film on a big screen---so I can see little incidental details that might be overlooked on a small display. Unfortunately, the 3D presentation of Avatar makes it inconvenient and uncomfortable to let one's eyes wander around a scene.
I decided to look around and see if anyone else had similar observations, and indeed they did: