You will see a resurgence of power leveling services, item trade services, and gold farming. Classic will have a fresh economy meaning for your average gold farmer time spent in classic is way more efficient. It will probably be worse than it was back in the day. Just look at the blizzard group finder on retail, littered with advertisements for m+ boosting websites. Disgusting. There will be so many gold farmers blizz probably won't be able to keep up. And this is what I truly fear because if the gold farmers start to win activision will step in and introduce wow token. And classic will die.
Travel Time is what you will be doing the most of, thus talents or abilities that increase movement speed will likely have the highest impact on leveling speed.The more important concept is this: Shaving off seconds of downtime between pulls is equally valuable to shaving off seconds of In Combat Time. When you think about it this way, Damage Mitigation and Regenerative talents / abilities / stats have almost offensive properties and should potentially be valued equally to stats that increase damage.
Most people who have spent years playing massively multiplayer online games will tell you that there's something special about the first one you played, too—and WoW was the first for many people. You can become so swept up in the uniqueness of your first experience that you overlook many flaws. It's likely many look at WoW Classic with rose-colored glasses, just as many simply forgot which features were added when. And some of today's WoW players may be too young to have even played vanilla WoW as it once was.