I probably don't need to tell you that really good virtual reality would be really cool. Your imagination will readily supply amazing things you could do with it, from training to video games. There's only one small problem standing between us and the immersive virtual future: violent nausea.
Inside your head is an amazing system for keeping you upright. Your inner ears detect motion, and your brain coordinates that signal with your eyes to tell you where you are and how your body is moving through the environment. However, if this signal goes haywire -- if the message from your eyes doesn't match the message from your ears -- your brain responds by instructing your gastrointestinal system to prepare for some vomiting. That's what causes motion sickness (and why focusing on the horizon, which tells your brain you're moving, often helps). It's also a symptom of conditions such as vertigo.