WoW was accessible in a way that previous MMOs were not: instead of repetitive grinding from the very start players are hand-held through levelling with the quest system and a story line. The levelling dungeons have comparatively generous static loot tables, with the rare drops from those dungeons just a nice little bonus. It's not until endgame that WoW reverts to genre.
WoW IMO killed Star Wars: Galaxies, which was exploring a more "open world" style of game; subsequent expansions progressively dialled back the classless system and player generated content.
More traditional repetitive grind MMOs remain popular in some parts of the world, but it's either EverQuest style or WoW style MMOs now.
I loved Star Wars Galaxies, that was the last MMO I really played.
I beta tested SWG -- it had so much potential.
You are getting the replies because though unintentional your prior wording implies that MMORPG was some niche experimental, concept type genre until 2004 which just doesn’t jive with history. EverQuest was out for years already and was immensely successful as was UO. That WoW became dominant is a different point. And as you have already stated - WoW didn’t do too much new in terms of genre defining.
Before WoW, there were a lot of different multiplayer games. After WoW, there was almost only WoW and WoW-clones (often down to mimicking the art style).