I was totally OK watching TM. Sonic didn't give me any nausea but flashes of headache.
When you walk around in life or drive a car, you really see the surroundings move around a heck of a lot. But your brain compensates for this and it will appear that you in a way is outside yourself and move in a fashion that the surrounding is still. If there is a movie shot from a person's view, they use a sliding camera to keep the surrounding still, because that is how our brains see it. If we see an amateur shooting a video, the video appears horrible and wobbly and unnatural, even if that is the raw data fed to us while we walk.
First person shooter is a crossbreed (usually) between a professional movie shot with smooth movement (the equivalent to the movie is games where you see "yourself") and the moving surrounding we really see while moving before the brain compensates. The surrounding moving and it being a game, not reality, doesn't at least let my brain compensate for the movement around me.
That is what makes me sick.