One of the most amazing new features of iOS 7 is that it's built on top of a physics and particle engine, allowing almost any and every element to come alive with movement - shifting, colliding, bouncing, floating, and more. It looks and feels like a game space. And therein lies a problem for some people. It's reportedly making them motion sick.
I have a friend who can't play 3D games or watch 3D movies on big screens for more than a few minutes without feeling sick and having to stop. Another with Labyrinthitis that's, off-and-on, threatened his ability to get around and even stay standing. I myself can't read for long in a moving vehicle without getting nauseous and having to stop. None of us have had a problem with iOS 7 to date, but for those who do, it can be a show-stopper level problem. Craig Grannell has been tracking the issue closely, and has written about it on Revert to Saved: