To gain gold in World of Warcraft, you will primarily be grinding professions which are fantastic for receiving rewards. Mining is an example of a great profession at an early-game level. While Herbalism is actually the best gathering profession, the issue is it uses consumables which can make the profession rather costly, with the investment required.
While I don’t know how meaningful the number will be compared to the number of players there will be at launch, having the sub requirement gives you a better idea that everyone who signs up is probably serious about playing. If everyone can make characters for free then there will be a lot of people that aren’t actually gonna play and meaninglessly inflates their numbers. This also presents the bullshit problem that somebody that never intended to play takes your name without having ever subbed. Which also introduces the thing that would 100% happen (and still probably will tbf, but less likely and slightly less sheisty in any case) that people will claim names to sell.
Loot everything - Every single item in the game has some value and you should take advantage of this fact. Grey items can be vendored for that little bit of extra pocket money and white stackable items are needed for crafting, which means that people will generally buy them if you post them on AH for low enough price (if you can't be bothered to post Auctions, you can just vendor all of them and be done with it). Some of the looted items will rise in value over time, so you might also want to take your time and store them on Bank Alt.
Whelgar's Excavation Site - This Dwarven excavation site was overrun by Raptors (Mottled Scythemaws and Mottled Razormaws). These reptiles are a great source of Medium Leather (you should have Skinning Profession if you want to farm here, as a very large portion of income that comes from this spot is acquired by Skinning Raptors). They also drop some Vendor Trash Items and Raptor Eggs (these are convenient, as a Cooking Recipe for them comes from a Quest that is located just above the Site) that can be used for Leveling up Cooking Skill. However, the best part about this spot is that both Raptor types present here drop level 19 Twink Gear (Feet of the Lynx, Sentry Cloak, Redbeard Crest, and Twisted Chanter's Staff) that will achieve astronomical prices at the later stages of the Game (drop rates are very low, so some RNG is involved, sadly). Recommended Level - 25+.