Picture what it would be like to play in a huge cardboard box and you'll understand what the level design is like in Deus Ex.
Yes. It's a cardboard box. A big fucking empty, open cardboard box. As opposed to Doom or Quake where you're on rails at all times. DX uses SYSTEMATIC level design. It employs a large open area, sets rules, and allows the player to choose their own path through it. That is what makes Deus Ex the greatest game ever made.