Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.
Note: My goal here was to create a comprehensive list of everything you'd need in order to learn to play any class (from Level 1 to Naxxramas)—meaning I am only including class-specific resources here (so no profession guides, general leveling guides, etc). Teeb's Resource list is much broader in scope, and I highly recommend you take advantage of it (as it contains a plethora of non-class specific resources as well). If you're not aware, Teeb is also responsible for the Race & Class Comparison Tool, Dungeon Tier Sets Gallery, and a huge variety of other insanely useful resources.
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.
It’s important that you levelling in certain areas around your characters level to gain the full XP reward for slaying monsters. Below is a list of all the best location for grinding XP from level 1 to 60, these locations are good for AOE spells and should have plenty of mobs. The Level you should be before you enter that specific location will be indicated to left in a Blue Color for Alliance [Lvl 35-39] and Orange Color for Horde [Lvl 35-39] the Mobs you are looking for Bolded [Mob Name].
2. Achieving. There are many pet battling achievements in game, and following the natural trail through the Taming [Continent] achievements up to Pandaria, and then on to the Spirit Tamers, Beasts of Fable and the Celestial Tournament was an epic journey for me and for many others. It's well worth doing. The Celestial Tournament is still, IMO, the pinnacle of PvE pet battling. However, WoD and Legion and BfA have added more battling achievements.
Before work began on World of Warcraft: Classic, it was only possible for players to experience the original World of Warcraft by using private servers, which are illegal, often have stability or corruption problems, and generally are very imperfect recreations of the authentic World of Warcraft experience. As much as Blizzard has been aware of the desires of their community, until recently it seemed impossible for them to emulate Classic servers due to the technical hurdles of essentially having to run two massively multiplayer online games side-by-side. A breakthrough was then achieved that made it possible to run Classic servers on the modern architecture of current World of Warcraft servers.
Another way to make gold. This option works well at level 60 when you have some good gear already. Obviously, you should have some gold ready to buy reagents. Be ready for expenses up to 225 skill points in any profession: items below that are not worth much. You will get first income after 225 skill points in Leatherworking, Blacksmithing, Alchemy or Tailoring. Unfortunately, Engineering and Enchanting will still require some investments. The only way to make gold with enchanting is disenchanting skill (we talked about it).