Good Map Design
Levels are filled with secret doors and passageways that give you more powerful weapons, health and armor bonuses, and extended ammo capacity. The levels are huge and it's easy to get lost (which is part of the fun) in some of the maps, and you always have places to explore and secrets to find.
The designs of the maps and the ways to clear them are all pretty varied too, DOOM is far from a typical corridor shooter with scripted events.
Some of the maps even have secret exits which take you to a different, secret level. A lot of these secrets will tease you and show you that there's a strong power up somewhere, but you have no idea how to get it. So you end up combing the entire level for the way to find the secret.