Gamespot has put up a small interview with id's lead designer, Tim Willits. While GS server seems to be getting hammered pretty hard, I still believe the short Doom III Q&A is well worth a read, this short paragraph should get you started when Willits was asked about any memorable moment during actual gameplay during tests...
I've had a number of great experiences recently, but one of the best happened just the other day and really brought together a bunch of the cool new elements in Doom III. I was making my way through one of the more lab-type areas of the game and started down a hallway. As I got about halfway down the hall, one of the zombie scientists approached from the next room. I was low on ammo and health and wanted to lure him down the hall, away from any other enemies in the next room, so I retreated to the previous room and waited. As he moved down the hallway, the light behind him was much stronger, so he cast a long shadow that moved ahead of him--I could see him coming just by his shadow. When he was finally in front of me, I shot him square in the chest with the shotgun. He just happened to be standing in front of a window to another room, and the force of the shotgun blast blew him back through the window and down a short set of stairs. We utilize "rag doll" physics on our characters, so his body tumbled perfectly down the stairs to the bottom. It was one of those moments where you just say, "Wow!"