The unnamed protagonist of Doom 3 is probably actually called "Marine Transfer", considering that is what appears on the passenger list at the start of the game (alongside Elliott Swann and John Campbell).
Classic Doom, the Arachnotron and Spider Mastermind have, compared to other monsters of similar strength, a disproportionately high chance of flinching when they're hit and being interrupted from shooting back. They're giant brains on mechanical legs. Of course they're going to be less resistant to pain.
Which has a bit of fridge logic in itself, since a brain is the ONLY part of a body that cannot feel pain at all (it processes pain but actual harm done to the brain itself can't be felt)
Possibly it's not actually pain, but unintended spasms caused by the punishment being rained upon them, then?
Also related is the fact there aren't any dogs in the secret levels. One wouldn't expect animals to be subject to the same punishment as the people that trained them.
And considering that they're dogs specifically, chances are they got a free pass.
Furthermore, notice how only the SS Officers are present, but not the regular soldiers. The soldiers, or Wehrmacht, were just ordinary people fighting for their country. The SS, however? They orchestrated most of the notorious war crimes of World War II. They are definitely going to Hell for their actions.
This is a common misconception, as the Wehrmacht committed more than its own share of war crimes. However, since the SS tended to dabble in the Occult (hence the commonality of The Ghostapo), chances are these SS goons you encountered did something to give themselves some degree of freedom when they went to Hell. So they could build their own fortress and keep the demons out (or tame some in the case of Pinkies).
The final boss of Doom II is John Romero's head on a stick obscured by a giant image of a Satanic monster called the "Icon of Sin". Appropriately, it is the source of all the monsters you've been fighting up to now, which of course is John Romero's duty as a game designer.
People complain about the huge, clunky PDAs used in Doom 3 as an example of zeerust when they should be using something closer to a smartphone, but it makes all kinds of sense when you consider the setting and context: It's an interplanetary mining operation involving joint private and military efforts, so they would issue their employees a heavy-duty, hard-to-misplace device with a highly visible screen and long battery life at the lowest possible cost.
The fight against the Cyberdemon at the end of Doom 3 is a call back to the final boss at the end of Doom 2. A small area filled with continuously spawning enemies and a boss that can only be hurt by one kind of attack.
The reason why the original cover's demons are nowhere to be found in any of the games is because Doom Guy already exterminated them all. DOOM 2016 strongly hints towards this when you find out that Doom Guy has been committing Demon Genocide since time immemorial, to the point where he used to wear medieval armor while doing so.
In 2016 Doom, the demons speak of a demon that created the armor for the Doom Walker. If one looks at Doom Walker's armor, it looks like a standard Space Marine Armor (Complete with production number and logos of manufactures). If this is the same armor that the demons speak of, why does it not look like some kind of demon armor. The reason, its a call back to DOOM 2. Rememer that John Romero was the real enemy of the game, he even says (backwards) that you have to defeat him to win the game. He helped create the game in real life, and is an enemy in DOOM 2. He is the demon who created DOOM Slayer's armor because he made the game. Makes the Demons warning seem kind of meta when one thinks of it.
In 2016 DOOM, it is mentioned that the UAC exploited Hell in order solve a major energy crisis. Since the game is implied to follow the continuity of DOOM II, it can be presumed that said energy crisis was the result of mankind having been driven from Earth and forced to quickly colonise new lebensraum, while suffering from a lack of preparation and resources.
Or since the Vault-Tec logo appears on one of the doors in the UAC facility, the energy crisis could refer to the Resource Wars and the Great War from the Fallout series. Bonus points for the game taking place in the 2140s, which would be seventy years after the Great War, and roughly twenty years before the original Fallout.
Doom has been frequently accused by Christians of promoting Satanism due to the abundance of demons and Hell being a setting of the game. But Doom is all about destroying demons, and invading Hell itself. Therefore, Doom is actually as Christian as you can ask a video game to be.
Levels set on Earth become a lot more disturbing when your realize that the demons had to make Earth fit them. Meaning that they horribly scarred the land, and slaughtered hundreds, if not thousands of innocent civilians to make it that way. And who knows what kind of reality warping powers they used to turn the landscape into the... Abstractness we see. And well, they're demons.
In TNT: Evilution from Final Doom, the forces of Hell invade via a spaceship. Looks like another race somewhere opened a hell portal, too.
Word of God has confirmed that Doomguy is the son of Billy Blaze. This means that in the Doom II bonus level Grosse, he not only found four lynched clones of his own father, but he was forced to kill each of them in order to progress through the level.
Following up on this, what happened to the Yorps and Gargs, the natives of Mars from the first Commander Keen game? Demons were unleashed on their moons and if they reached the planet's surface, they'd be defenseless. And that's if the UAC didn't do anything to them first.