For me it's 3 things:
Number one is that we'll all be starting from square one. When the servers open everyone will be a level 1 with 0 gold or anything else to their Classic WoW name. The only advantage people can get is through grouping up with others.
Making gold will mean something. Gold inflation has been over the top for a while now. Making gold used to be as much a part of the game as getting year. Now it's more of a collection for big numbers thing. I remember just killing mobs for half an hour or so as I leveled so I had money for repairs/talents etc.
I did the Quel danas dailies just for the gold in BC. And all the dailies on the isle only game ~ 120 gold. Just doing a world quest now with my follower gives me 125 gold. It will be nice having that whole aspect of character progression up again.
#3, leveling will be part of the game again. Leveling alts will go from something I dread doing to possibly something I'll want to do over end game stuff. I miss actually caring about my gear as I leveled, having some pieces for 20-30 levels. With travel around the world being more difficult and leveling taking longer, towns in random leveling zones become hubs. People hang out there like they do in cities now. Leveling feeling like part of the game and not a barrier to the rest of it gives the game a fundamental base it's been missing since ~ Wrath.