I spent most of my life afraid of roller coasters, though I could handle "kiddie" ones like Barnstormer, and Primeval Whirl. Then my parents started talking about moving from Orlando to Costa Rica. I didn't want to leave Disney World and was very upset. Finally, Dad said he would stop talking about it if I went on The Incredible Hulk (Islands of Adventure) with him. I agreed. And I found that scary rides weren't so scary after all. The thing I've dreaded most about coasters is what Goofyernmost described as an "empty stomach" feeling. I don't like feeling that way. But most coasters, even when going downhill, don't affect me that way. Except for Space Mountain, because I can't see the drops ahead of time. Yes, the Hulk is less scary than Space Mountain, for me, anyway.
For me, the scariest part in most roller coasters isn't the drops, but the long climb upward at the beginning of most. The last time I rode a coaster, the red side of the now extinct Dragon Challenge at Islands of Adventure, I wasn't scared, but I did feel sick on it and for a while after it.
My fave coasters combine roller coaster and dark ride, and have no lift hills to climb upward at the beginning: Rock'n'Roller Coaster at DHS, and Revenge of the Mummy at Universal Studios. Though nothing can prepare you for going from zero to sixty at the beginning of RnR, not even having been on before!
I have yet to go on Tower of Terror, though. Since that's a thrill ride that goes straight down, I'm 95 percent sure I'll get that empty stomach feeling on it. I was going to go when they first opened it, but I was scared and then I realized theme parks are meant to be fun, not stressful, so I skipped it.