Having been able to see it both formats, I found that I was able to shift my gaze and focus on something in the foreground or background in a well positioned imax seat. The other 3d format (RealD?) was far more locked to a chosen foreground object it seemed to me. YMMV

I felt the same way in the Imax. And to provide some context to my downvoting detractor, here's what I was using as the basis for my speculation: http://news.ycombinator.com/item?id=995096

It's a huge comment, so here's the relevant bit:

"What Cameron has been doing with Avatar is to shoot in deep focus (no using the aperture and focus controls to blur out the background, a favorite technique for isolating the subject from the environment) but instead create depth by altering the angle between the two lenses dynamically, creating the illusion of a large space in which attention to depth is focused stereoscopically. Until now most 3d projects have kept the stereoscopic distance fixed, which yields the feeling of watching the story take place on a stage in front of one and occasionally having one of the props or actors protrude outwards toward the audience. By varying the angle between the lenses in the same fashion as our eyes, Cameron presents a far more immersive way of experiencing the third dimension."

Thanks for digging that up! If I understand this, it's not just that there is simple depth of field in the 2D/photography sense where the background of a primary subject is blurry. They are literally forcing your eyes to look at the primary subject using 3D tricks as well. This certainly would explain why I had such a strong sensation that I was being forced to focus on specific things... because I literally was being forced to.