The Horde 12-20 (Barrens + Stonetalon Mountains) guide has been rewritten and revamped.  In addition, there has been numerous tweaks to the speedrun route to make things faster and easier to follow.  Also, in case you haven't noticed I am now adding about 40% more info per step and redoing all the images.  Unlike with my previous vanilla guides, I am now giving a brief explanation of where to go and what to do with each step along with any other helpful tips.
Within the context of Classic, the method most frequently proposed is to re-tune bosses to make them harder. Rather than copying the numbers, re-create the relative difficulty. Kind of like, when you first start working out, you use small weights. When you get stronger, you lift bigger weights. The WoW community is more knowledgeable, more experienced, has better hardware, etc. We're "stronger." So give us tougher bosses. There are a lot of forms that could take. Bosses were constantly re-tuned during vanilla anyway, so if Ragnaros does 10% more damage or something, is that really a problem? I don't think it is. What about resistance gear? back in the day, collecting resist gear was a thing people did. But the numbers and strategies are so well known today that only the tank really needs it anymore. Bosses could be re-tuned with more resistible elemental damage so that's important for entire raids to gear appropriately. That would be entirely keeping with the spirit of vanilla even if the actually numbers sitting in a database somewhere were different than they were ~13 years ago. If you want to go a little farther, a few people have proposed adding new abilities to old bosses, adjusting their timing, etc.

A new quest will pop up once your first pet reaches level 3. It will send you to either Audrey Burnhep in Stormwind or Varzok in Orgrimmar. These trainers will offer a quest chain to battle specific Pet Tamer NPCs on each continent. Defeating each of these NPCs will reward a nice chunk of experience for your pets, as well a Sack of Pet Supplies (which has the chance to drop the Porcupette companion, among the always-useful pet bandages).

Most profitable professions are considered to be Herbalism and Enchanting in pair. You can have Enchanting only for Disenchanting skill, it will bring decent gold. Consider the fact of disenchanting all soulbound armor, not just selling them to a vendor, but I do not recommend to disenchant any kind of weapons, it is more profitable to sell it to a merchant if it is a soulbound item, or sell it on auction house if it is BoE (bind on equip).
Earning gold in World of Warcraft: Classic is all about optimizing your spendings. Back in the day, your character’s skills were acquired from a trainer, using money. Don’t just buy everything he or she offers! Read the skill descriptions and be mindful about them. Quite a few characters can live through the levelling process without learning some of their utility or damage spells. These won’t go anywhere and you’ll be able to learn them later on.
Nov 5 Yellow names I got killed by an undead character with a yellow name. I know some npcs are green and players on your side are blue. Is this an enemy player or an npc? I thought the undead and blood elves were on the same side but I think it might have been a player because he/she jumped over and over again on my corpse after I was dead. I'm really confused by this.Lithelilara12 Nov 5
In OSRS, before bonds (their equivalent of tokens) were released, there was a strong gold industry, but most people who bought gold did so by buying bonds on the main game, then finding a dedicated 'swapper' who'd exchange their RS3 gold for an appropriate amount of OSRS gold (taking a cut for themselves, of course). I wonder if a similar system will pop up in classic wow
The quickest way to level up is to travel through the game's zones, completing all the quests you encounter along the way. Completing quests grants you Experience points, which will slowly fill in your experience bar with blue or purple. When the bar fills, you reach the next level. Alternatively, you could choose to run Dungeons or Battlegrounds, which are not covered in this beginner's guide.
It is a great advantage to start making some silver and your first gold coin at low levels already. Skinning is a nice choice here. Fast and easy way to gather resources, leather of all kinds while leveling for those who are at the race to level 60. There are many good spots to grind beast mobs and skin them. I like doing it as a Troll Hunter, trolls have a passive racial ability: + 5% bonus damage against beasts. 

To get Blacksmithing to level 300 is much effort. It is not that thing to make fast gold. There is always a lack of Blacksmiths though. Enchanters need rods of all kinds, melee classes look for Sharpening Stones and Weightstones with Counterweights, others buy Iron Spurs. Once you have learnt rare and epic recipes, people will always use your service to craft items.
Much like an encounter with a wild pet team, PVP Pet Battles are turn-based. However, unlike the PVE version, the PVP combat system is timed and any ability for each round must be chosen within that time limit otherwise it will assume you have passed a turn. A time penalty is applied if you take more than 15 seconds to choose an action, resulting in progressively shorter round timers for you.
Each battle that you win will yield experience for each pet that participated. However, deceased companions will not receive any experience (so make an effort to keep them alive by swaping in a different pet). Experience gains are based on your pet's level compared to the wild pet. Defeating a higher level opponent will grant more experience than a much lower level one, but be ready for a tough fight!
Where to get materials and reagents you may ask? Buy for cheap off farmers in bulk or track Auction House lots. Get in touch with most big sellers. Communication is important here, send a whisper or write them a letter. It would be good for both of you to establish business relationships. Constant suppliers and buyers is convenient for both parties to save the time and effort.