Yep, Space is a roller coaster. It's more of a "wild mouse" coaster that doesn't have the steep drops, though- more the swooping and banking around. The fact that it's dark actually helps a lot of people who don't like regular coasters, because you don't have that sensation of looking down the steep drop in front of you and screaming all the way down. It just looks like you're swooping through the stars. As far as actual thrills and drops, it's not rated very exciting by coaster fans, because it's not that intense, but the fact that it's in the dark makes it more exciting, and makes it too intense for some people. On the other hand, I've heard of some people who don't like roller coasters that actually love Space.
Splash isn't a coaster-- it's a flume ride, in water, in a "log", but it does, of course, have a few drops. Mostly it's a gentle ride through a story, where you are looking at all the scenes and singing, then there's the big drop at the end. Which, in my opinion, is not half as bad as it looks from the outside. You can get splashed a bit, or you can also get really soaked, depending on where you're sitting, and how you hit those bumps!
Big Thunder Mountain Railroad is definitely a coaster. It's fairly wild, and it has a lot of very tight turns.
Bear in mind that this is all subjective-- real roller coaster aficionados scoff at DL's rides except possibly California Screamin', because compared to the big ones, they're not that scary or intense. But for many people, they're too much.
I like roller coasters, though I haven't been on very many and not any really scary ones. But I LOVE them all at DL! I will tell you that I have motion sickness issues with rides like gravitron, zipper, octopus-- anything that goes round and round while spinning (I barely survived the Teacups

) But I love the rollercoaster rides! DD5 loved them, too, as did my niece who was 5, and who went on Screamin' 3 times in a row!
HTH--
