The roller coasters at WDW aren't bad. They aren't the out of control, high speed, many inversioned roller coasters at Universal or anyother local amusemant park.
The 2 "worst" ones are the new Expedition Everest and Rock n Roller Coaster. Both are pretty fast, but aren't too intense.
Expedition Everest, you go backwards at one point, but you just do some helixes and turns, EE also doesn't have an loops. EE just has one big drop, and it isn't even that bad. It's just a little steep, but it's over with in about 1 second.
Rock n Roller Coaster is a launched roller coaster. It go to about 60mph. But after the shoot off, the speed drops. It's still fairly fast, but not too bad. Plus you are inside, so it's not nearly as scary. This ride has 3 inversions. 2 half loops in the beginning, then a corkscrew near the end. They go by quickly, and you have a harness with handles to grab onto if needed. Plus this ride is in the dark. You can see the track, but I think the dark adds some comfort, so you can't see what's coming so you can't get scared for it. This is my favorite ride at WDW, and it's a lot of fun.
The rest of the roller coasters are very tame. They only go about 30mph and there aren't any more looping ones. Space Mt. at Magic Kingdom is in the dark, and you can't really see the track infront of you. It has sharp, sudden turns, but it only goes about 28mph. There aren't any big drops, just small dips occasionally. If you want a less turbulant ride, try and sit in the middle of the train. The train seats one person per seat. So you sit behind the person in front of you. The people in the very front and very back have the most intense ride, in the front you can't see a thing so you get jerked around more (if you sit near the middle, you have about a second to brace yourself for the turn) and the back you get whipped around the corners.
Big Thunder Mt. at Magic Kingdom is like a kiddie roller coaster. It's still a lot of fun. Just turns and a few little dips. It also goes about 28mph.
And the funnest of them all, Primeval Whirl. This is in Animal Kingdom and it is a spinning coaster. It doesn't have any huge drops, but maybe one decent sized one. It's still like a kiddie coaster. It's not fast, but you car spins during one part of the ride. So when you go around the hairpin turn, you start spinning. It's sooo much fun, as long as you don't get motion sickness!
If I get scared on a roller coaster, they do have the restaints to hold onto! That helps. Or even holding onto the person next to me, as long as I know them.

And if I know what the ride is like, then it helps a lot. If you'd like to see what exacly happens in the rides, there's a site of videos I found.
http://www.mousebuzz.com/forum/mouse-trap/25310-lot-disney-videos.html Some of them are from
Disneyland, but most of the rides at WDW are here. The first time I went on some of the bigger rides at WDW, I was scared, then when I watched the videos of the ride and got an idea of what happened, I felt better and I could go on the ride with little nervousness.
Have lots of fun, and if you need any more info on rides at WDW, go to this site
www.allearsnet.com It has tons and tons of info and pictures!!