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Pretty much this. Especially now we have editors like DoomBuilder- Most other engines simply aren't around long enough for their tools to reach such a level of refinement and ease of use. Stuff like Quake and Unreal Tournament wasn't much more complex in theory, but Doom is still the only game of a reasonable complexity where you can read a tutorial, and start cracking out perfectly decent maps, in the same afternoon.
Even I have managed to make a reasonably impressive set of Doom levels in my time. I could just never figure out how you do stuff like changing the title screen and background music etc in order to make a cohesive pack out of them. If I could have figured that out I was going to put the .wad on my band's last CD release for shits and giggles, each level would have a midi version of a track from the album and then there would be a little text scroll about how a humble roadie saved the tour from hellspawn.