I played the vanilla wow back when it launched and through each expansion. I'm quite clear on what was good about it, for me, imho, it is a mix between thinking back nostalgic to the day's when I was 18 and it launched. My life outside the game was great, and all my friends played the game on the same server.
Mix that with the fact that it was -new- and I got to explore and discover stuff for the first time. Those are parts of what made it great, but I've already experienced it, so I feel no need to go back. Kinda a whole "Been there, done that." situation.
However, this might make my voice not count at all, but I feel that should they ever add Vanilla/Classic back on demand from players, it should be as it were, not a single thing from any era outside of the classic. Like add the realm/server to the last big patch, the Drums of War (1.12) with the crossrealms battleground system. Then leave it there. Lest it won't be "Classic". With that patch and that system, they'll have plenty of stuff to keep their hands full. The endless grind for PvP titles, the plenty of raids, even original Naxxramas and such.