To me, vanilla WoW was about choice. I understand there was demonstrably less to do back in ye olde days but hear me out. I've never been a raider. Never will be. I just don't have the time or the will. It makes most MMOs pretty... dull for me nowadays. I like the ones I like and I stay subscribed to FFXIV but ultimately I feel like there isn't really much to do; I'm just grinding out gear so I can see the next raid so I can see the next raid, etc, etc. There's a reason most MMO players say that glamour is the real endgame. It's the only thing to really accomplish outside of raiding most of the time.
Back in vanilla I felt like I always had something to do that didn't have much of anything to do with raiding. Sure, the gear treadmill still existed but there was farming for gold so you could buy your mount. There was farming for mini-pets because they looked cool. There was exploration because without a mount the world felt massive. Hell, doing dungeons with people was fun and exciting because you weren't always there for the drops--or at least not drops to get you raid-ready. Blackrock Depths had quests associated with it that gave
blue gear and that was exciting! You got to do quests! Purely inside of dungeons! It never felt like the game was 100% balanced around getting you into a raid to see it and the MMOs I've played (FFXIV & WoW) are completely balanced around that. You log on, do your daily checklist and... probably log off. There isn't a lot of interesting stuff to do unless you're raiding. I can't come home and shoot the shit with friends while grinding away in Feralas to maybe get a hippogriph mount or a sprite dragon pet.
While there was less to do, it felt like I had more options for what I wanted to do. Part of that is definitely because everything was a very long-term goal, but I stand by the fact that most modern MMOs are mainly about ticking off the boxes so you can gear up so you can get into the latest raid.
It was also about shit like this:
https://www.youtube.com/watch?v=uhb2m7eLgYk
In order to start a raid, raiders had to do a questline that had them walking through Stormwind with a coterie of NPCs to uncover one of the primary NPCs as Onyxia, the boss of the raid, herself. It was awesome to see and it got your non-raiding player engaged with the raid and what was happening in the world. Shit, Naxxramas came with a world event that gave cool rewards to people who never set foot in the raid! Some of this extended into Burning Crusade.
...Fel Reavers, man. I remember being so goddamn hype to roll a blood elf paladin. Burning Crusade was also amazing.
(I know GW2 and ESO are more along the lines of that non-raider deal, but I can never get into them for some reason.)