All of the Daikatana and Ion Storm jokes aside, what Romero was able to achieve from a level-design perspective in 1993 with Doom, given that such an engine had never existed before, was pretty mind-boggling.
I wish I had a chance to take game design classes with him now that he's a professor at my alma-mater (UC Santa Cruz), especially after seeing his walkthrough of Knee-Deep In The Dead on YouTube recently[0].