I was recently interviewed by a Vancouver radio station about my book on Expo 86 and was asked what makes these events so special. The first thing that came to mind was the architecture, for fairs provide a rare opportunity to go wild with fanciful building designs. I remember being just amazed at the odd shapes at my first fair, the 1964-1965 New York one, and the same feeling of wonder has hit me at all of the fairs I've been at since.
Here's a view of the Germany pavilion at Expo 67, for example. You're just not going to find something like this anywhere else!