I just spent the last week in St. Louis, MO. I was on a business trip, but I always try to see one or two things whenever I travel to a new city. In this case, a couple of my co-workers were very gracious and took me to see the St. Louis "Gateway Arch".
It is hard to explain what impressed me about it. It's a very simple shape (A catenary curve, to be precise). Its size is certainly impressive, but there are larger structures, and certainly more difficult ones to build.
I think it was the elegant simplicity of it. Looming over the hodgepodge of architectural styles that make up the St. Louis Riverfront, it stands out by being a very pure expression. Many of the more intricate works lose something by their complexity.