Well, I have finished all 4 episodes of Doom and beat Doom 2 on Ultraviolence mode without codes.
I started the 1st episode on Nightmare mode today...and its freakin insane....
not because of the enemies respawning...but because Doom feels like running at about 200 fps with XP...
And at 200 fps its pretty hard to dodge an imp's fireball....and the demons move freakin fast....
So with respawning enemies, which is normal for nightmare, combined with 200 fps....its freakin insane......
*Disclaimer: No real fps rate is known, 200 is the estimated amount, and it is probably overestimated. If you know of a way to find out the actual frame rate for Doom, please tell me*