Why is classic so good? It depends
It depends on what you enjoy. Do you enjoy being "fed" something? Or do you enjoy "building" something.
OG wow centers on leveling which is a drawn out process which is not "hard" in a skill based way, but it is "hard work" in that it's a real time investment. However, you are rewarded in a way that is satisfying as your character is "built" in contrast to modern games where you are "fed" rewards.
Because there are competing things that people enjoy, the game was re-steered towards the "feed me" crowd, which is probably a bigger audience. After all, that's what people said about wow post everquest etc.
I'd argue that the game was partially "ruined" as early as burning crusade.
No more sprawling 5 mans like uldaman, gnomer, BRD, ST. All essentially converted into hub types like SM. The devs remarked on the success of SM but I think the metrics were wrong. A place like BRD is a real dungeon crawl. Almost nobody remembers hellfire ramparts with fondness. It's fast, efficient, get in get out, but that defeats the purpose of the mmo genre. If you need a quick 30 minute in and out because you're a busy adult it's the wrong type of game to play.
I'd essentially say a lot of gamers don't know how to enjoy themselves. Dopamine hits have ruined them. The OG game worked because devs did what they thought was fun, not what would be most efficient so people could tick boxes.
It's all subjective though but I'm glad the OG game was designed in the same way that I gain satisfaction, through building a character over time. That said, you maybe can't do the whole leveling experience a second time hence the just +10 levels in BC etc. It's kind of a one off thing.
TLDR
Leveling and long dungeons made it great. Things taking a while to achieve makes a bigger payoff when you finally manage it. Being fed constant rewards devalues the rewards!