A History and Analysis of Level Design in 3D Computer Games (2006)

Level design was kinda fun back in the day of Quake/Half-Life/Unreal. Professionals or amateurs with enough skill can push out a good looking level in a few weeks to a couple of months. A good team of 6-8 amateurs can pump out a full map-pack (usually consists of at least 10 maps) in under 6 months.

Since the introduction of more sophisticated rendering engines level designing (and asset creation in general) has been pushed further and further away from modders or even professional themselves.

I'm not particularly sure whether this is a good thing or not, because nowadays people are still making maps and mods for legacy engines such as the first DOOM, first two Quake, the two Half-Life and UT99 (Unreal Tournament happened to contain all UnrealScript for Unreal and Unreal return to napali thus becomes the premium development platform for Unreal single-player modders, plus it has a better engine and more importantly a better editor). But when you go down the stream, the more modern the engine is, the more difficult to make mods for.