Not all of WOW is very compelling and I wish the author had taken his admittedly unscientific ranking and examined different zones to see what would come out on top. Especially in Vanilla there were a lot of grindy areas and depending on which faction you chose you would have a radically different early game experience. The Orc, Tauren, Dwarf, and Undead starting areas I remember being pretty boring and it seemed like a lot more care was put into the Night Elves and Human areas.
The best environments were generally in the mid game where the developers seemed to have the most leeway in what they were building theme wise. Once you got to the end game it became very grindy and the environments were not so compelling. Everything became volcanic, lots of generic evil and vistas of browns.
Having said all that - I think the real reason it was so compelling was the community. It hit at just the right time where everyone was finally coming online and there weren't many other games of the same quality level pulling people away. It was a well designed game and everyone seemed to be playing it. I only spent so much time there because it was where I could reliably chat up friends. The few people I know who are still playing it today only do so because of their guild.
I remember the first time going into Ironforge, and being immersed completely like nothing else before or since.
Even my first Oculus did not beat that.
I'm not sure what it was. The graphics were far from ground breaking. There were other games with big worlds. But I think the combination of world building, graphics, scale, and quality of execution (nailing the style) just completely shattered the level of anything else that came before - or at least for so many people that played the game.
The reasoning for the difference in complexity of the Alliance vs Horde starting areas (and lategame areas) in Vanilla was discussed somewhere (don't have a source handy). The artists and other creators started with the Human and Night Elf areas and created the elaborate areas you reference. Once they realized how long it took to do those areas and that they would never be able to meet the release deadline if they took the same amount of time and effort for other areas it became much more of a copy-paste job.
Indeed, if you ever pull up the earliest alpha screenshots where WoW is recognizable as the game that got released, the very first zones that were worked on were Elwynn Forest, Westfall, Duskwood, and Stranglethorn Vale and that was very apparent in Vanilla.
It's bizarre how well I remember that game... It was the design of each area, the music, ambiance, how the quest system introduced you to an area, but there was exponentially more to discover if you explored. Then the hook were instances where you couldn't realistically go solo, which led to pickup groups, which led to frustration with randos, which led to elitist groups forming which became top raiding guilds.
I met some insanely talented and accomplished people who moonlighted in raiding guilds within WoW. Friends raiding, writing custom mods, while working at the top tech companies. Such a time sink, but so much fun.
Oh.. and do you remember theorycrafting? Reverse engineering the game mechanics and optimizing for each situation across all the classes to beat raid bosses that were designed to be nearly impossible to defeat at launch? That was incredibly interesting, especially in a 40 player environment with multiple classes. The depth and balance of that game was and is still unparalleled.
Some areas were also just unfinished. The stories/quest-lines just simply ended. For example Dustwallow Marsh suddenly ends, and while you do go back that for one of the first raids (Ony) it is largely disconnected from the zone itself.
WoW Classic had this stuff in spades. Areas that were masterpieces (Tanaris, WPL, EPL, Human/Dwarf starting, et al) and areas (and ideas) seriously neglected or forgotten. Most of the content added in patches was fairly high quality though, they just seemingly focused their attention on that end-game instead or revamping since "most players have levelled passed that."
My first character was a Night Elf. You can never capture that first experience of playing WoW. Starting out in Shadowglen and growing to the point I could reach Darnassus is simply the best gaming experience I've ever had. Then you realize that you're just on a tiny island and there is an entire world to explore beyond what you have just witnessed. I never thought you could experience actual awe in a video game before that. And I have never had that experience since then.
I'll never forget when I managed to squeeze my Night Elf between the trees that made the wall around Teldrassil, only to realize that the whole map I had been exploring for so many hours was the trunk of a humongous tree, and the sea was far below.
Having said all that - I think the real reason it was so compelling was the community. It hit at just the right time where everyone was finally coming online and there weren't many other games of the same quality level pulling people away. It was a well designed game and everyone seemed to be playing it. I only spent so much time there because it was where I could reliably chat up friends. The few people I know who are still playing it today only do so because of their guild.