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.
When it comes to Classic, Blizzard's goal is to provide an authentic Vanilla experience, but they also needed to come up with technology to handle many players at launch, which is similar to sharding. While the use of this technology will be limited to the first month following launch, many players are questioning this decision and wouldn't like to see layering in the game at all.
Now that you know to loot everything (see tip #1), you’ll have a lot of items in your bags. Any item that is white quality or greater may sell on the Auction House. How do you know what will sell well on the auction house? Trade goods and green quality (or better) items will almost always sell on the Auction House. See our Auction House guide (coming soon!) for more information.
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.
Classic WoW is the highly anticipated, upcoming (re) release of the original, first World of Warcraft version, known as Classic and by it's other popular ice-cream like name: Vanilla WoW. Dubbed Retail Classic, or Official Classic by the playerbase - it was finally announced by Blizzard in November 2017, after a lot of pressure from the Private Server World of Warcraft community, since the latter's shutdown of the most popular Private Server called Nostalrius which featured the first WoW version in two separate PvP and PvE servers.
The priest was considered the best overall healer in vanilla WoW. Priests had the biggest selection of healing and defensive spells which could be used in various different situations. Due to this fact, Priests had an easy time finding groups and guilds. If you’ll embark on leveling priest, expect some random whispers from other players inviting you to join their group as a healer. Shadow Priests were also considered as one of the best duelists in the game boasting with serious damage and healing.