Just being powerless, really.
Certainly other fears play into it, like heights and drowning, but generally I want to feel like I can do something if anything were to happen. Like, I climbed Kebnekaise, which is Sweden's highest mountain (though really, it just felt like a high pile of rocks) and when you're up around the top, it's something like 2000 metres up, which is rather scary. Then my cousin wanted to go to the actual highest point, which is basically a long, really thin (about 1,5 metres wide at most) and I just went "Fuck that shit," and sat around in a cabin and waited for her. Just a small gust of wind could send you over the edge and it's a long way down to have your life flashing by.
As a young kid (think I was somewhere between 5 and 9, hard to remember) I remember being on vacation on the coast and walked on a rock-bed near the water, when I stepped on a wet spot and fell in. It was just so scary, knowing that if the stream was faster than it was, I would have just gone further out into the water. It really freaks me out when I think back on it.
I'm the same when it comes to vehicles. Obviously aeroplanes are a big issue for me, especially when departing. On a recent trip, I found myself wanting to crash over the ocean, so we wouldn't get smashed to bits on land. Same with cars. I get really tense and uncomfortable when someone else is driving, especially one of my friends who is at times very reckless in traffic.
Never ever would I be caught doing that nightmare-climb Ross posted.
For the guy that said cows: