I’ve been looking forward to this game ever since E3, but I only made it about 15 minutes before I started to feel seriously ill.
GHofPersia, via Gamefaqs

VR Comfort Mode is an optional beginner mode that just so happens to be super comfortable to use in practice. Not everyone will love it, but its far less immersion breaking than running to the couch for a glass of ginger-ale 🙂
CloudHeadGames, via Reddit

Simulation sickness occurs due to some of your senses telling your brain that one thing is happening, while other senses are busy telling your brain that something else is happening – a sensory mismatch between your visual system and your vestibular system. When your visual system says you are moving but your vestibular system says you are stationary.

Simulation sickness is significant issue with VR, as the visual effects are so persuasive. The symptoms of VR induced simulation sickness can be severe, ranging from mild discomfort to being partially incapacitated for up to a day two afterwards, even in some rare cases left with permanent effects. A bad first experience can turn someone off the VR completely, perceiving the issue as being entirely with themselves or the platform, rather than game-specific.

Many considerations are shared with non-VR games, such as avoiding any difference between camera movement and head movement, or ensuring an appropriate FOV is set.

Others considerations are specific to VR. These include, but are not limited to:

• Locomotion options that avoid a sense of movement that doesn’t match player movement; such a teleporting, and snap-rotating in 30 degree increments
• Keeping the camera in full control of the player at all times, and always controlled by head movement by default (although providing an extra option to use controller instead is helpful for motor accessibility)
• Dynamically reducing amount of peripheral vision during movement
• Giving the player a frame of reference (such as tracks or a cockpit)
• Maintaining a constant horizon
• Avoiding objects quickly moving towards/away/past the player (including large objects moving close to the player’s head)
• Avoiding acceleration/deceleration
• Maintaining constantly high framerate (minimum requirements currently vary depending on platform and research – 60fps, 75fps, 90fps – but it should always remain constant) and low latency (below 20ms)
• Motion blur effects are problematic outside of VR too, but for VR pay particular attention to avoiding blur in peripheral vision
• Avoiding requirement for fast head movement, to reduce risk of the headset shifting during use
• Use of language, for example talking about comfort rather than illness, as just having the idea of simulation sickness in your head can make you more prone.

Some of these considerations may clash with your intended experience. If so, still implement them, but treat them as your default (“comfort mode”), with the option for players who know they can handle them to turn them off.

More information: Oculus best practices: simulator sickness
More information: VR & Accessibility
More information: Simulation Sickness and VR – What is it, and what can developers and players do to reduce it?
More information: How to Avoid the Effect of Motion Sickness in VR
More information: This is Your Brain on VR (video – 0:23)