Have you ever seen a movie about space flight, where the astronauts ride in a flight simulator?
I'm specifically thinking of the movie the 'Right Stuff." though the one they ride in the movie is not enclosed.
Remember the scene where John Glenn (Ed Harris) turns green because the simulator last too long/goes too fast. Mind, the ride isn't as bad as that, but the structure of the ride and the simulator in the movie is roughly similar. Mission:S Orange is basically a mechanical flight simulator. You car is enclosed, on the end of a long arm. (I should add here, a number of rides are called "motion simulators" but they don't spin like M:S. M:S is much more like a real flight simulator because of all the real motion. the green side just tilts. Star Tours is just a box that tilts. M:S Orange riders feel real force.) I think maybe the flight simulator in the movie, "Miracle on the Hudson" also showed a flight simulator.
Other poster is correct, some folks feel ill because their brains can tell the real motion doesn't match the kind of movement that is depicted in movie.
Other folks feel fine, and for them the ride simulates what it would feel like to be propelled in a rocket.
The CM's tell riders to FACE FORWARD the whole time. If you turn your head, you are more apt to feel the mismatch and get ill.
I've done both sides, and well, both orange and green are a bit boring, IMO. Personally, while I get the other poster was trying to say, IMO M:S is quite a bit different from the HP Hogwart's castle ride.
The other reason M:S Orange got a bad wrap was because some years ago a child died after riding it. I couldn't tell you the exact specifics, but the child had a pre-existing but unknown medical condition in his brain. Something about the ride triggered a reaction that caused his death. this was back when the ride was still relatively new and it received a good deal of publicity at the time.
If you happen to go on the new HS Smugger's Run ride, well...iMO it has some similarities to M:S, except a bit better. It doesn't spin, but every rider has an actual role/impact on the outcome. Mind, SR still didn't blow us away, but it is more fun than M:Space, with less risk of throwing up.
As I said eariler though, many people are fine riding M:S. If you are prone to motion sickness, it might not be for ou. If you are fine with similar rides, try it.
When M:S first opened, some of the first to ride were real astronauts, who said to them, it felt like a real blast off, and -compared to other rides of the day - what their NASA flight simulators feltlike. (as compared to tilting box type rides like Star Tours.)