I've played WoW since its day one untill the end of Cataclysm as an hardcore player into several top ranking guild on the Italian community.
Vanilla was amazing because everything was new, almost no one had the knowledge and all the world and istances were there to explore. The mechanics of bosses were pretty boring and simple if you look at them backward, things get more serious with AQ40 and Naxx but MC and BWL are just time sinking holes (and the majority of players cannot even overcome them).
The real issue were you needed a massive guild to make progress: 40-men raids requires at last 60 active members constantly on par with equip, knowledge, consumables and the like.
For a casual player's point of view, imho, Vanilla was pretty terrible with tons of content locked out because it requires too much time for farm what it was needed and most of all a decent, big, group of other really dedicated players.
Many people that are overly excited for those Classic servers will probably find out how plain flat was that and how much everything was simple overly time-consuming. The real fun was experiencing that massive world with many of the amazing communities that were born back there.
Something that now will never be possible again.