|Facing Worlds in Unreal Tournament.|
|Minas Morgul in Doom II.|
Nuts3.wad has over 12000 monsters in UV and is a huge challenge to complete. Doom3 has maps with only a few monsters in it and a new monster like the Revenant is announced with a cutscene where it flaunts its abilities and then you fight it. Doom did not do that, the monsters would just be something you would run into and you had to be prepared to fight by conserving your rockets and other ammunition for the impending battle, like E1M8 for example.
This blog post is an in-depth analysis of the level architecture in the Doom & Doom 2 maps and is a very interesting read. I recommend everyone who is into Doom give this a read.