Map making in Doom was (is?) so accessible as the editors were in 2D for a 3D world. Granted you couldn't place 1 room directly above another room, but no one cared too much at the time. Once things moved on to Quake a lot of my friends lost interest in trying to "figure it out" as the editor was more challenging.

In high school a group of friends would all create their own maps and then before classes started we would play test them on the computer lab LAN. Our favorite editor at the time was DEU.

None of us being map making geniuses, we'd always end up with something that didn't play well but did have a few "fun spots". Eventually someone had the idea to just cherry pick all those fun spots and Frankenstein together a single map using our individual parts. This map ended up being the most fun we have ever had in Doom.

I'm not sure if Doom was the first, but it was the first for me to instill the idea that a game should be moddable if you want to make a great community and give the game a very long life after the developers have moved on to the next thing.