Of those who actually stomach the grind to max level, most won’t actively log in and play the way they do on live servers, because there’s basically nothing to do at that point, certainly not when held in contrast to today’s game.
You won’t be playing through big engaging ongoing storylines, because the original release of WoW had almost no story or character development to speak of, certainly far less than any of the last few expansions, as notoriously limited as they have been in that regard.
You won’t be coming back consistently to work on daily quests, because those won’t exist.
You won’t be hanging out working on achievements, because those won’t exist.
You won’t be collecting pets, toys, or transmog gear, because those systems won’t exist.
You’re unlikely to be collecting mounts, because there will be relatively few available, and they all take up a chunk of your very limited bag space.
You won’t be quickly queuing up and teleporting to dungeons when you have a few minutes free, because there’s no group finder for those — or any activity for that matter — so you’ll need to be ready to slow-travel to every drawn-out dungeon after manually assembling a group for decidedly non-mythic experiences.
If you’re a serious raider, you’ll spend more of your time in-game literally just grinding for the stuff necessary to even complete the raids than you will in the raids themselves, and the abysmal early raid design and miserly loot system will see people quickly abandon raiding, to the point where just finding 40 willing and capable people will soon be the biggest challenge.