Heights. Like most people here I can't stand more than 4 stories (any place)
If there are railings higher than the hips, then I'm fine with 4 stories outdoors. Otherwise, 2 stories max.
In fact, a story:
Two weeks before the WTC got taken down, my family took a vacation there. It was real nice and stuff. I get to one of the observation decks (89..ish floor?) and I say to myself, alright. Let's see the view. I look from the slanted windows (you can lean slightly against them) and I started to freak out because I thought the windows couldn't stand my weight. I was fine, I got away from the windows. Next floor higher, more obsevation deck. Vertically oriented windows this time. I look, and out of nowhere, my sister jumps out at me from behind when my nose is right at the window. I was 13, and I cried for 5 minutes.