No datamining. We didn't know what secrets the world did or didn't hold.
No flying, the world didn't feel small because you couldn't AFK fly across it in 2 minutes.
Nostalgia, it was for many, our first MMO, our first experience.
For many of us who were younger, we had way more free time to just play and have fun.
Any other answers are rose-colored glasses, nostalgia fueled misrepresentations. You were ALWAYS treated as a hero in end-game, not some nobody. We killed Onxyia and Nefarian and were celebrated in the capitol for it. It wasn't the group finder, it was becoming a huge problem and wasn't fun to hang around a summoning stone while a friend flew to Stormwind or Org to spam the LFG chat, and it was even worse by WotLK. PUGing through it was a huge highlight of WotLK.
Don't listen to idiots who don't understand why their experience was better. The devs were experimenting and trying things. There were unpopular additions and changes in Vanilla and BC, not just now.
The major problem modern WoW has is this stupid systems that occupy much of the game (Garrisons/Order Halls/Artifacts) which are annoying, enhanced so by the fact we know they're going to be useless next expansion.