I would describe each of them like this:
Space Mountain: A fun, but generally tame, roller coaster. It's fun because of the darkness. It can be jerky, so if you have any soreness, bad back, injuries, etc. that could be aggravated, I would sit it out.
Rockin Rollercoast: I call this a fun and exciting coaster. It's never really scary, except for the few seconds before it launches...but they give you a tiny bit of a countdown, so you can prepare yourself. Once it's going it's just pure fun and excitement. This is my favorite ride at Disney.
Everest: This one is the Thrilling and "scary" coaster. They use the theme well, and the slow climb up the hill is typical and builds suspense/excitement. The ride itself can also be intense, especially the backwards part.