Unrelated to the world design, but another factor in the success of WoW that I didn't see mentioned is the rich add-on ecosystem that would allow you to deeply

Unrelated to the world design, but another factor in the success of WoW that I didn't see mentioned is the rich add-on ecosystem that would allow you to deeply customize the UI. I believe this programability was an important factor in the popularity in the tech circles. High level play almost required you to use various addons (boss mod, threat meter, dps meter, raid frames etc). Spending hours tweaking and coding to make sure my UI was setup to give me and my guild an edge was a big part of what made WoW fun for me. Also, the fact that you could record and export detailed combat log data was another unique feature of the game that made it appealing to number crunching geeks.