As everyone already knows the community/multiplayer element of classic WoW greatly adds to the enjoyment of leveling, unlike retail leveling where the path of least resistance is pretty much always to play solo. However one of the main problems that I have with classic leveling (other than it taking too long) is that it's not multiplayer enough.
Throughout my journey to 60 I only felt obligated to group with players around me for difficult quests and "Kill X of these" quests. But I feel that if grouping was the path of least resistance for all types of quests by a significant margin that would greatly improve the leveling experience overall.
Imagine if you didn't have to gather double the items for the gathering quests, and your xp wasn't cut in half when grouping with 1 or 2 players. The community element of the game (in regards to leveling) would thrive. For every quest players would be actively trying to group up with other players in general chat, and every zone would be filled with players looking for other players to play with. It would be so easy to make friends, and it would make the leveling experience so much easier for the people that hate leveling.
The other benefit of this system is that you can keep leveling difficult, so that the people who actually enjoy solo leveling can still do that, and a small reward for doing so would be the gold that you make (since gold is split when ur in a group).
The problem with retail leveling is that its so mind numbing that only bad players can enjoy it, but with this system bad players could simply group up and romp through the quests, and the portion of the playerbase that actually enjoys leveling (beyond it just being an AoE massacre like retail) wouldn't feel alienated.