[fh]

Even infinite depth of field wouldn't completely solve the problem. The point is, your eyes have to physically focus on the far away screen the entire time, or the whole movie becomes blurry (obviously). When you're looking at a "near" object, there's always a mismatch between its apparent distance and the focus point of your eyes. And as you pointed out, you can't turn your head.

However, given these limitations, I've found 3D movies to be a great experience. In the case of Avatar in particular, an important point of the movie was how the protagonist becomes part of the alien world, and I felt that the 3D environment really helped the audience to get the same feeling. YMMV of course.

What's really a rip-off is all those movies now that were converted to 3D in post production. It's a bit like manually coloring a black-and-white photo before color film was available -- it may look kind of okay if you don't look too closely, but it's not the real thing. The flat faces in particular make people look like cardboard cutouts.