Content
Never played vanilla back in the day, so most of the content is somewhat nostalgic, because I know most of it via later expansions, but also somewhat new and fresh aswell, since I never experienced it at the intended extend and power level.
Immersion
The world of vanilla WoW just seems so deep and thought-through. You see a certain location and know exactly where it is. Also the lore in the background has this typical "Warcraft" flair, which counts among my most favourite variants of fantasy. In context to other universes, it is kind of down-to-earth and tries to explain itself, to a certain extent, without beeing rediculous. I can only suspend my disbelieve for *so* far, lately alot of works overdid it or didn't even try at all. It has a certain myth, too. Most stuff I already know but not in and out.
Gameplay depth
The general gameplay in vanilla is by no means perfect. Most people may argue that most specs play really awkward or clunky -at best- and often make little sense. The balance in most places is just not there, at all. But still, it can be so much fun. It is not about managing your actions bars, spamming buttons and filling EVERY SINGLE global cooldown (because that is apparently fun and gameplay done right) ontop of intricate rotations and bulky class mechanics. It is about actual gameplay in a group environments and managing certain aspects of your class and/or role. Sure, most specs are just plain brain dead in most situations, esp. PvE. But the difficulty and the effort you have to make to actually succeed and get rewarded is still there.
By no means is it terribly hard or advanced academics, but it can still be demanding and the actual difficulty lies in basic gameplay mechanics that complement the multiplayer aspects of the game. As a healer you have to manage your mana efficiency to keep people alive, as a tank you have to manage aggro, which can be quite the chore but still interesting if multiple targets are involved. As Dps you have to manage your resources, positioning and general aspects of the individual ranged or melee play style.
Everything mentioned above can of course be trivialized in the context of actual difficulty, but I hope you get my point.
And most classes actually feel different and diverse, have their own unique roles with stronlgy pronounced strengths and weaknesses. And yes, the *good* is not distributed evenly among the classes, but oh well.
Multiplayer aspects
Like mentioned above, vanilla WoW actually feels like a game with multiplayer in mind. The stuff you can do solo is mostly limited to grinding, crafting and basic questing. A huge part of the game happens with/against other people. You need them, they need you. Some may argue that you get forced into group content, I just say it incentivizes playing together in a way, every multiplayer focused game should.
This aspect also reinforces immersion and investment into the world, because most challenges you face you can't possibly tackle alone and thus the world feels deep, huge and epic. How it should.