I've ridden Glen Helen a few times at the vet nationals, using 1/2 to 2/3 of the big Mt. St. Helens. Every year I am shocked again by how rough that place gets on race day. I have never seen anything else that even closely compares. The bumps in the straightaways get bigger than our whoops down here in Tx. and they are everywhere. It's impossible to imagine until you've ridden on it. And after you have, you get a whole new appreciation for the pro's.
Jason81 is right on about the SX tracks. In 99 a local pro raced the Dallas sx, and I was his mechanic. I couldn't believe how different it looked when we walked the track than from the stands. The faces are vertical, and the whoops looked unrideable to me. They were pyramid shaped, 3' deep caverns! It would be like hitting 30 three foot tall curbs at speed.