I think I'm like a lot of people who are scared of heights. I just can't do the lean over thing or look down past my feet, etc. Then I get the whirlies.
I'm fine in a plane or a gondola or on a chair with the bar down, but lift the bar up and I get all wierded out. Funnily, I've been ski-diving and hang gliding and loved it, but sitting on the edge with my feet out the plane's door was not fun.
The worst, though, was an old 8th floor apartment with only 30 inch high railings on the balcony. Man, I hated going out there!