This is another one that will most likely be a boon rather than a bane to the player base, though time will tell just how 14years of experience will affect the economy of Classic WoW. There has been no concrete word on just how AddOns will work in Classic, but if the infrastructure of the game works the same as it does in retail, there is a good chance that most of the mods that work in retail will work in Classic. This means that quite a few people will be running around with a whole host of gathering, crafting, auctioning, and gold making mods. Now, those mods did exist back in Vanilla, but not in the same way they do now, and not as many people had them back then as will in Classic. This will drastically affect how effective the auction house will be, and hopefully will affect the economy as a whole in a positive way. Another thing that will most likely see a large increase in popularity is carry runs. These have steadily grown in popularity since Vanilla, and rest assured with the old 40 man raid size that there will be quite a few “Molten Core full carry master loot ON PST for prices GOLD ONLY” being spammed in trade chat. Whether tokens will be available in Classic has yet to be discussed, but if so it will have an enormous impact on the economy of Classic. This, in addition to the differences I will cover in the next section, will have a pretty large impact on the endgame of WoW Classic.
One may think that the AH may suffer a reduction of competitivity, leading to higher prices, but in an healthy server the gold farmer's farm will be taken over by actual players. Giving the chance to a more fair gameplay where you can go out and farm the materials that you need, instead to see them always farmed by gold sellers that sell them to you, without letting you many room to make gold in a fair way.
Things like Tauren melee reach being bigger, which is something I experienced going away live during Black Temple Illidan progression (at least for that particular encounter), is confusing to players as it's a significant advantage over other races in some instances. No quest tracking is another apparently confusing issue, as many Mankirk's WIfe seekers well know, and there's a whole lot more where that came from: