But for about 6 months there if you were lucky enough to have been playing at release, Azeroth was a beautifully-realized, insanely vast and deep open world, full of more interesting things to do and see than any other game experience I've ever had, before or since. The reasons are many, and you were either there and know what I'm talking about, or I'm just another idiot yelling at clouds on the internet.
No, no, I totally understand you. And I had a very similar experience the first few months I played WOW. It was actually a combination of several factors that together created an experience that is difficult to repeat, but I would say essentially for two main reasons.
The first is that Warcraft, at that particular moment, still had an interesting lore for many people. Yeah, Warcraft has always been somewhat generic, but many of the early WOW players grew up along with the franchise, playing Wacraft 1, 2, and 3. It's true that for many Warcraft 3 already proved to be a "detour" (both in mechanics and theme) from the series, but at the same time, you can find the roots of WOW there. So the first point is that, for many people, WOW was the first moment they could literally experience what it would be like to venture into the... World of Warcraft. It was literally seeing the whole world, the whole fantasy universe that you could only see in stories and tiny maps before, open to be explored. I think that for the overwhelming majority of the fans of the series, that experience was something incomparable. And it's something that hasn't been repeated in other open world games.
The second point is the matter of novelty. The first experience, or at least the first experiences you have with an open world game are quite remarkable. When you have not yet been exposed to the concept, the initial feeling that you can walk "anywhere on the map" is extremely impactful. Especially if you are a person who was used to playing more linear games. I imagine that the impact must have been much less remarkable for those who had the chance to play games like Daggerfall before. But I believe that for many people, WOW was the first open-world game that seemed "real", in a fully 3D environment, with a camera that you could control. You really felt you had a whole continent to explore. I remember the feeling of seeing a cave and going into it without any transition, it made the game seem much more like a "real place" to me.
Other open world games were also fun in their own ways, but most likely, my experience with WOW was so impactful because of the whole context. I was much more willing to forgive the game's various problems/flaws because I was so excited about the world and the exploration it offered me. Grinding for hours is something I find unbearable, but even that became more tolerable in WOW when I knew that by getting more levels I could explore different places. Especially because the way the game was initially created, you felt like an adventurer going on your own adventures into a big world, not someone being carried away by the nose following a predetermined story.
But yeah... You can never experience the same thing for the first time again.