Since about version 1.6 of Minecraft, I know of several people including myself that suffer from instant motion sickness when playing minecraft.
I'm almost 100% certain that it has to do with the video rendering. In most situations it is clear that the video rendering produces a distorted image. This becomes most evident when the player stand in a 90 degree corner in a canyon, and very slowly turns around. It can then be clearly seen that the walls of the canyon move in a non-linear motion. It is this distortion that most likely produces the motion sickness.
For instance, I'm able to play serveral first person games such as Oblivion or Skyrim for hours continued gameplay with no motion sickness at all, including movements in "tight spaces". And on the other hand, some older First Person games that had render "optimizations" and thereby also a non-linear distorted view, produced the same motion sickness.
The result is that I had to stop playing minecraft altogether, which is a shame.
Could you consider adding a render mode that produces acurate rendering? Most likely that would prevent motion sickness and enable people sensitive to it to play again.