What are the differences between the difficulty levels in Doom and Doom 2?

Question

I've been playing Doom and Doom 2 for a while and have grown curious if there is an official list of what changes when you increase the difficulty level. I've noticed that there are almost always more enemies, and they sometimes have increased firing rates. But are there any other differences regarding enemy hit points, enemy intelligence, item placement, enemy regeneration, etc.? What are the major differences between I'm too young to die, Hey, not too rough, Hurt me plenty, Ultra-Violence, and Nightmare! skill levels? Is the difference in overall difficulty pretty much constant between each one, or is the jump between Ultra-Violence and Nightmare! much larger than that between the lower levels?

Answer 1

The difficulty jumps quite often rely on the map you are playing on and vary quite a bit between maps. Here is a general guideline of the difficulty levels you should expect. The information is taken from the Doom wiki. For difficult level on individual maps, it's best to consult their corresponding wikia pages.

Skill Level             Monster numbers   Ammo amount   Other
I'm too young to die    Least             Double        Player takes half damage
Hey, not too rough      Least             Normal
Hurt me plenty          Default number    Normal
Ultra-Violence          Most              Normal
Nightmare!              Most              Double        Monsters respawn, faster attacks, no cheats allowed

Source: Doom wiki.