Two types of gameplay depth in games:
Depth of Choice
&
Depth of Possibility
Possibility x Choice = Total Gameplay Depth
When you confront a lone Hell Knight in Doom 3, you are usually confined to the room or you can choose to retreat back into a previous room (sometimes). Therefore, the choices you can make are limited.
The depth of choice is in choosing the optimal path of progression. Sure, you could try to punch the demon to death, but the game makes this hard and it would take a long time. I don't even know if you can attempt it... the hell knights always position themselves to face you and the fist is a useless weapon if you want to avoid collateral damage.
In the old doom games, one could position two different monsters in line so as to make one shoot the other, and then you watch them fight. I don't even remember if this was possible in Doom 3... the corridors are to tight for such strategic maneuvering and the monster spawn spread was usually limited to 1 type of monster or 2 types of monsters with wildy different speed or height variables or melee attack styles.
The depth of possibility in Doom 3 is therefore lower than the depth of possibility in the older Doom games, quantitatively. The depth of choice is still there, but many of the choices that were beneficial in the older Doom games lead to sub-optimal results in the new one.
Total gameplay depth in Doom 3 is significantly lowered because of the missing possibilities and the hollow capacity for choice that follows from that. Not only is it less deep, it is also frustrating.
An example of this frustration arises from the use of the Doom 3 berserk powerup. Unlike the old games, Berserk now makes you invincible, and it is only available in "Berserk-friendly mazes", where the mini-game goal is to merely find and punch all of the enemies in an area before time is up.
In previous games, Berserk power-ups were rewards for exploration, and they allowed you to conserve some ammo if you dared to go up close to the swarming hordes of monsters. Since Doom 3 has no swarming hordes, id decided to change the function of berserk I guess. Depth of possibility and choice were removed because hordes were not part of the game design.
All of Doom 3's problems really do stem from the lack of monster hordes. A real shame they removed that integral Doom element.
edit: improved definitions:
Depth of Choice: The options and capabilities given to the player (movement, attacks, special moves) to overcome problem areas. Some choices you make in Doom 3 are move, attack, look, open, retreat, shoot, explore, collect, dodge, and etc
Depth of Possibility: The open action pathways through a situation, made by selecting an option or capability given to the player. Example possibilities in Doom 3 are strategies for dealing with monsters like shotgunning Imps in the face (most optimal), choices to make in conserving ammo, clearing areas smartly, taunting enemies through choke-points, or simply running around the room avoiding the Hell Knight swiping attacks. Level design directly affects possibility. The more properties and characteristics monsters, items and the environment possesses, the more possibility arises as long as interactions between these characteristics and properties are numerous and complex.
Emergence: The more options and possibilities there are in a game, the more depth of gameplay emerges.