3D technology can create stunning movies, but many people dislike wearing 3D glasses. Most 3D movies rely on a technology called stereoscopy, which requires those glasses to function, and viewing a movie without wearing them can have unpleasant side effects. Under the best circumstances, the quality of the movie will suffer a little. Images will become blurry and some discoloration will occur, but watching the movie is still possible. In other cases, watching a movie without the glasses can make the viewer feel ill, so viewers should be cautious about leaving the glasses off.
Most 3D movies create the illusion of depth by displaying two separate images. The 3D glasses are designed to force your brain to treat the pair of images as though they were a single image, and it does so by creating a 3D picture. The human brain cannot do that without the glasses, so most people who take them off will see a pair of overlapping movies. The overlap may be more or less noticeable depending on the precise technologies that the movie uses, but it will almost always be present to some degree.
Many directors also use color to help separate the two images in the movie. This usually take the form of a red outline around one image and a blue outline around the other, which is why 3D glasses usually include red and blue lenses. The colored lenses will hide the outlines, but they appear quite clearly to people who keep the glasses off. This puts another level of distortion in the movie, and it can make it very difficult to see what is happening.
The distortion and discoloration that appear when a 3D movie is viewed without glasses can cause minor medical problems. At an instinctive level, the human brain understands that blurriness and strange colors indicate some sort of visual problem. Unfortunately, human instincts have not adapted to modern technology, so the brain will assume that the problems come from poison. Motion sickness works the same way, and people who are prone to suffering from motion sickness while they are driving are particularly likely to experience this problem. When it happens, the brain triggers an anti-poison mechanism and induces nausea. In extreme cases, this can lead to vomiting, although most people will only be uncomfortable while they watch the movie.
Many movies, especially in the horror genre, are designed with 3D viewing in mind. When they are not watched in 3D, they can often become confusing or lose quality. For example, many horror movies depend on sudden 3D images for shock value, and most of their horror will go away in 2D. That means that even a movie that was created with 3D in mind and that lacks any doubles images or discoloration will still be worse when viewed in 2D.