Yanger's Quick Rundown for Returning and New Players:
Welcome to my quick guide to help you get back into the swing of WoW! This is going to focus primarily on Warlords of Draenor, I'll try to note where those things that are changing from the current climate are so that you don't get too confused if you're already interacting with those systems.
We'll start with the super basics, and then we'll move on to endgame activities. Every section will have a brief rundown for when you quit the game how it has changed, if it doesn't list anything for your era, it hasn't changed drastically.
Classes
Whether you're new or returning, classes in this game have a lot of diversity and it may take you some trying to figure out mechanics and which one fits you the best, I recommend just trying what sounds fun initially. As much as we still use the word rotation, there is no such thing as a 'rotation' in the traditional sense anymore. Everyone has priority systems. If you ever feel like you're just hitting 2 buttons, you're most likely doing it wrong (Or the class is very badly tuned for low levels, but that hasn't been the case in my experience). Expect most complexity to be revealed sometime in the 70s or 80s.
If you Quit in Vanilla or BC: Class Balance is an entirely different beast than it used to be. Everyone can DPS and do it reasonably well (Even at a raid level), there are many viable tanks, many viable healers, in general the capabilities of the classes are closer than they ever were back in the day. Class complexity is vastly higher than it was back then (The more complex classes like Rogues and Warlocks are the norm rather than the exception).
Nearly every class has more complex resource management and sometimes multiple resources to manage now.
If you Quit in Wrath: Most classes have more complex resource management than when you played, all specs are at least somewhat viable compared to Wrath.
Death Knight: Tank, DPS. A two resource class with Runes and Runic Power, you spend one resource to build the other, and lots of procs. They are no longer 'can spec anything for any role' as they were in Wrath and now Blood tanks, Frost is a fast paced actiony dps spec, and Unholy is a methodical thinking man's DPS spec.
Druid: Tank, Healer, Melee DPS, Ranged DPS. The only class that can be any role. Resource management depends on the class, tanks use a semi fast paced rage build and dump setup. Melee use Energy and Combo Points like Rogue (And are noted for their John Madden complex rotation). Ranged DPS use a sort of ebb and flow Eclipse Bar where you cycle through lunar and solar phases and use different spells to move it from day and night. Healers still focus a lot on HOTs and Area healing.
Hunter: Ranged DPS. Overhauled completely during Cataclysm, they no longer have Mana and now use a resource called Focus, which regenerates on its own but you actively can force regeneration by using a filler spell (Steady Shot/Cobra Shot). Pets can fill in any buff or debuff your group needs, making them extremely versatile. The three specs are pretty similar on a basic level, they just spend their focus in different ways. Warlords offers a talent to play a petless variant if you so choose, but it's probably not always going to be the best choice if you don't like pet management.
Mage: Ranged DPS. The three specs play slightly differently than they used to. Arcane cares a lot about mana management as it does more damage the more mana you have, but it also spends the most. It has a lot of peaks and valleys. Fire tends to be very streaky and crit dependent, if you like rolling the dice and seeing how you do (Within reason, there are safeguard mechanics to ensure you don't go TOO long on a cold streak) then Fire is for you. Frost is probably the most traditional playstyle, though it has a permanent pet these days (Water Elemental).
Monk: Melee DPS, Tank, Healer. The new class in Mists of Pandaria, I'm assuming most people using this guide haven't played one, so I'll get this out of the way: They are awful to play at low levels and have a VERY involved kit of abilities, the basic gameplay for tanks and DPS revolve around Chi and Energy, which is similar to a rogue, but it's faster paced most of the time. Windwalkers are pretty what-you-see-is-what-you-get. they've been challenging to play but mostly due to eccentricities of the class, but it is a lot of fun. Tanks have a LOT of abilities but it's all pretty well designed, and they've been the strongest tank for the entire MoP expansion due to their versatility and scaling. Healers rely on Mana and Chi rather than Energy, and were weak a lot of the time in MoP but when they shined were really strong. They have a lot of uncontrolled healing like healing spheres and Renewing Mists (Floats between targets to blanket the raid sort of like druids used to do with Rejuv, and then you Uplift and heal everyone that has it on them), but they also have a fun DPS/Healing hybrid that is being more fleshed out in Warlords to make balance a bit easier.
Paladin: Tank, Melee DPS, Healer. Paladins have changed a lot since the early days - Blessings are now seperated into Hands and Blessings, the Blessings last an hour and are just standard buffs you barely worry about, and Hands represent the shorter term more 'cooldown' type blessings, IE: BoP is now HoP. Justice is now a Hand. Salvation is a short term buff that dumps a LOT of threat on someone, things like that. All Paladins now use a Resource called Holy Power, which is a sort of combo pointish system. Tanks and Melee don't rely on their Mana very much, so Holy Power is their primary resource, along with the Cooldowns of their abilities. Building and spending Holy Power much like Combo points. Healing Paladins excel at single target healing (as they always have) and having very high survivability.
Priest: Ranged DPS, Healer. Priests are still the most versatile healers, as the only class that has two healing specs still. Shadow is a spec that I'm not really sure how it is changing in Warlords, but in MoP it tends to do low single target damage, and excel at multitarget fights. It's basically a spec based around dots and building Shadow Orbs that you then unleash into large single target nukes or dots. Holy spec is a standard healing spec, with a very diverse toolkit (Hots, AE heals, single target heals, strong cooldowns), and Discipline is a ridiculous spec based on shielding people pre-emptively, also with a lot of powerful cooldowns. Disc can also be a semi-DPS spec at times where their damage can heal, much like Monks.
Rogue: Melee DPS. Very similar to how they used to be in Vanilla or BC, but with a lot of procs thrown in to all of the specs to make the rotations more dynamic. Combat excels at cleaving, Subtlety likes to use large single hits, and Assassination relies on poisons. Poison itself is no longer crafted, and you get 1 damage poison and 1 utility poison at any time (There's just a 'poison' button that expands out to list them all and you click the ones you want). They are probably the best class at avoiding damage and other mechanics in the game, and they are almost always a top DPS contender. Hard to imagine a world where rogues aren't highly desired anymore.
Shaman: Melee DPS, Ranged DPS, Healer. Shaman play pretty differently depending on the spec you want, they are among the highest burst dps in the game however and tend to excel at burst dps phases of fights, or things like Challenge Modes. The melee spec is very fast paced, building and spending stacks of Maelstrom quickly and utilizing a procs. Elemental is a very strong cleave spec, working best on a medium amount of targets. Resto is still a sort of cleave-y healer, working best on clumped groups of medium amounts of targets, but also having some nice utility (increasing max HP of the raid when they're actively healing people).
Warlock: Ranged DPS. They've changed a lot over the past two expansions, all of their specs are much more differentiated than before and use different resource systems. Affliction is still heavily DoT based, they have a soul shard mechanic much like all locks had in Cataclysm: You gain souls from draining and use them to unleash more powerful versions of spells. Destruction is based around fire and huge hits - you gain Embers from burning your targets, and you spend them to do things like copy all spells (for cleaving) or unleash Chaos Bolts which are one of the hardest single nukes in the game. Demonology is based around Demonic Fury, which you build up through your normal rotation spells and then use to transform into a Demon form where you gain different abilities.
Warrior: Melee DPS, Tank. they still rely on Rage. Protection has a really strong kit with solid active mitigation, Fury and Arms are both being changed a lot in Warlords (And Prot is getting a DPS subspec), right now they rely on getting Colossus Smash up (you ignore armor for 6 seconds) and then unleashing as much damage as possible in that window.
Beyond the basic class changes, there are some general notes for roles:
DPS - Not much to say here. Kill stuff.
Tanks - Vengeance is a new mechanic (Replaced by a less broken one in WoD called Resolve) which is basically meant to let tank damage scale at the same rate as DPS do. Vengeance instead lets tanks crush DPS on most raid bosses, hence why it is being removed. Basically, the more damage you take, the more damage you do (With some limitations). Resolve is going to let you get the defensive benefits of Vengeance, without making it appealing to stand in fire to do more dps. That brings us to...Active Mitigation. You might hear this word some, all of the tanks now have a more active role in how much damage they take. You don't just stand there and get healed, you have buttons to press that aren't long cooldowns. Death Knights heal themselves and shield themselves with Death Strike, Warriors have a very short cooldown Shield Block that absorbs damage, as well as a Shield Barrier to burn excess rage. Druids have a rage dump to increase their dodge to astronomical levels. Monks stagger percentages of their incoming damage and then use Purifying Brew to remove the remainder (Which turns into a dot on them otherwise). Paladins spend Holy Power to either heal themselves or block incoming damage.
Healers - Mana is standardized now, which affects healers more than anyone else. Gearing up no longer increases your actual mana pool (In MoP it is ALWAYS 300k unless there's a talent or something that increases it). This means what actually improves as you gear up are heal efficiencies (they hit harder so you cast less) and your spirit which gives you regen. Many classes have active mana regen they can utilize, such as Mana Tea for monks or Rapture for Disc Priests.
Wow, that was longer than I expected, and perhaps more than necessary. I'll try to keep the next parts a bit shorter, the ones with actual systems.
