I can tell you what did scare me, though: rappelling. The rappelling tower at Freeland Leslie was no higher than the zipline tower, but I could never bring myself to use the former. I did try it once, sort of; I climbed the ladder to the top of the tower, strapped on the harness, and approached the edge ... and then stepped away, took off the gear, and went back down the ladder. I just couldn't bring myself to jump backwards off the top. Stepping forward off the zipline tower was never a problem, forward is forward and backward is backward. Having all that empty air behind me just freaked me the hell out.
Basically, what it boils down to is that I'm afraid of falling. Heights don't bother me as long as I can perceive and believe that there are measures in place to keep me at that height. But take that away and I kind of turn to jelly. That's why I couldn't rappel; even with the harness on, once I got to the edge of the platform and turned around, I didn't trust that I wasn't going to fall. Similarly, when I visited Yosemite National Park several years ago, I was hiking up to the top of something or another, and it was going fine until I reached a point where the guard rails went away and I was left on a trail with a sheer rock face on one side and a open air on the other. I kept going as long as I could, but eventually the altitude and lack of anything preventing me from succumbing to it got to me, and I turned around. Curses!