At one point in time, there was very little in the form of big attractions at Epcot. The Epcot park has never been meant to be an attraction, but Mission: Space does a great job of sticking with the "Experimental Prototype Community of Tomorrow" as it gives you a chance to get a simulated feeling of a space shuttle liftoff.
Not only does this space shuttle liftoff, but you get to travel through space while taking one of four possible roles of the flight team.
There is a motion side and a non-motion side for those that experience motion sickness at all. The motion side only spins to create g-forces while you are taking off or using boosters for the space shuttle.
The ride is a great experience, especially for those who love the idea of space travel!