IIRC, Doom had mostly graphical edits, but total conversions and other significant gameplay changes were popular with Quake 1, including CTF(and all its variants), Team Fortress(and MegaTF), and Rocket Arena.
Quake was definitely the first big one - the difference is that id had minimal contact with mod teams, and left people to their own business.
That's when Halflife stepped in, and it went from there. I think it had mostly to do with the fact that a ton of people bought Halflife because of all the buzz it was making, liked the graphics engine and then the mods started. Because of the communication that Valve had from modmakers, it started from there.
IIRC, Doom had mostly graphical edits, but total conversions and other significant gameplay changes were popular with Quake 1, including CTF(and all its variants), Team Fortress(and MegaTF), and Rocket Arena.
Many games before HL had mods. I remember Alien Doom and picking up a bunch of others on some 3rd party CD and Computer City. Duke Nukem had a bunch, as did Quake. Oh, no! The Romh!
ZZT (1991 Epic Megagames - developed by Tim Sweeney) was probably the first game I can think of that had extensive modding capabilities... later on, some people made some really cool stuff with it, even though it only had 16 color ASCII graphics I made a couple of games for it, none of them were anything too great tho
"An overhead-view adventure in the style of Rogue, using text-mode graphic characters. The real appeal of this game is the level editor and the 1000+ (yes 1000+) and growing worlds. This game, strangely, is still going strong. Can't say I can explain its appeal, but I can say that the game rocks."