> Chief of these was the level editor that he designed along with the engine, which, for the first time and surprisingly the last time, allowed a level designer to “walk” through the level in 3D as he was designing it. This WYSIWYG (what-you-see-is-what-you-get) mode allowed the designers to create levels in a far more natural way then they would be able if they were merely using lines on a grid.

Not was the last. Obvisuly, the guy that wrote this on 2001, not know UnrealEd. The map editor for Unreal engines have this capability. Eventually, even map editors for classic DooM would have it (Check DoomBuilder for example)