For the environment design aspect, I'd add that in the earlier iterations of the game something that inadvertently added a lot of appeal was the somewhat "organic" nature of zone composition, where there wasn't a 1:1 match between what the quest writers needed and what got put on the game map.
There were all sorts of things in each zone that just existed to exist, with no connection to any quest or player objective. It felt like the map artists were given a lot of latitude to design as they pleased, with the quest writers coming in after them and writing quests to match the map as it had been designed.
This was flipped on its head at some point around Wrath of the Lich King or Cataclysm, where instead nearly every square inch of every map existed for some quest or player objective. It made the game more convenient, but also made it feel a lot more like a game than a lived-in world.
The term for this is "conveyor belt content". And yes, this took off in WotLK, probably in big part due to the achievement system. Why? This replaced player agency in determining content with a Blizzard-supplied checklist and the effects of this flowed on to map design (IMHO).
It was also around that time that i think the Blizzard's internal policy changed.
Before to that they were creating a game, and balancing it in vacuum. Afterwards they went for designing metagame - picking how player should play. It was more egregious in their newer games though - especially in diablo3(where Blizzard literally makes builds in form of armor sets) and overwatch.
There were all sorts of things in each zone that just existed to exist, with no connection to any quest or player objective. It felt like the map artists were given a lot of latitude to design as they pleased, with the quest writers coming in after them and writing quests to match the map as it had been designed.
This was flipped on its head at some point around Wrath of the Lich King or Cataclysm, where instead nearly every square inch of every map existed for some quest or player objective. It made the game more convenient, but also made it feel a lot more like a game than a lived-in world.