Basic plot revolves around N'zoth, the Naga, and introduction to the void lords. Queen Azshara, Jaina, Alleria, Tyralon, Khadgar, and Mangi are all main characters in the story. A new continent is discovered on the other side of the world, hence the name "Dark Side of Azeroth." Along with this new continent, Najazatar, Kul'tiras, Plunder isle, and other islands are all new areas. New an mystical creatures and races will be discovered on the new continent (13 zones), some of them are transformed creatures due to Azeroth's awakening, and Azeroth imbuing them with power (some of the races siphoning power from Azeroth herself) This will give the expansion more of a feeling of adventure and exploration rather than just needing to fight another world ending threat (although we do still need to deal with N'zoth and the naga, and eventually the void lords.
Azeroth is in a weakened state from the happenings in Legion, and N'zoth, the naga and the void lords take advantage of this, giving them more power. While the heroes of Azeroth were busy on Argus, Azshara stole 2 relics including the tidestone, and the tear of elune. We will have to once again recover the 2 relics in the initial dungeons and raids of the expansion.
Main Features:
New Races:
Ogres (Horde) - Tired of sitting on the sidelines and watching the war with the legion and other forces, the Ogres decide to re-ally up with their former orcish and horde comrades. Warrior, Mage, Warlock, Hunter, Priest, Death Knight, Monk, Shaman
Naga (Alliance) - Outcasts of Azshara's armies, these Naga send emissaries to Kul'tiras to bargain with Jaina and forge an alliance with her. When Jaina calls out for the Alliance's help in safeguarding Kul'tiras from Queen Azshara's invading armies the renegade Naga are absorbed into the alliance due to their resistance against Queen Azshara. Warrior, Hunter, Shaman, Mage, Rogue, Death Knight, Monk, Druid
Etheral (Neutral) - Traders, gamblers, and refugees. The homeworld of the Etherals was destroyed and they have taken a particular interest in Azeroth and the dealings with its races over the course of several years. An opportunistic union with either the horde or alliance will serve to bolster their ranks and gain prominence in the cosmos once again. The Etherals choose to forge strategic alliances for the pure sake of profit and glory. Warlock, Mage, Priest, Hunter, Warrior, Monk, Paladin
New Race / Class Combinations:
Horde:
Tauren: Warlock, Mage
Orc: Priest, Druid, Demon Hunter
Undead: Paladin
Goblin: Monk
Troll: Demon Hunter
Alliance:
Human: Druid, Demon Hunter
Draenei: Warlock
Gnome: Paladin, Shaman, Demon Hunter
Night Elf: Warlock, Paladin
Worgen: Monk, Shaman
New Continent:
13 Zones
15 New Creature Types
9 Factions
Main Azeroth
3 New Zones at launch, more in patches (Kul'Tiras, Naz'jatar, Plunder Isle)
New Quest Type:
World Events - similar to world quests, world events are more organic and happen randomly within the zones you play through. You might be doing a world quest, and an event pops up nearby that is limited in time and after players complete the event, its gone until another event pops up. These are group events that you contribute to, and depending on your level of involvement in the event determines your reward on a Bronze, Silver, or Gold basis.
New Game System:
Path of the titans - similar progression system to artifact weapons, more details fleshed out at Blizzcon
New PvP Game Mode:
Free For All Death Match - Similar to the Gladiator Arena (building from Warlords), horde and alliance will enter this mode to get to a set kill count. Respawn timers are greatly reduced. There will be new maps for this game mode, (10, 20, and 30 player maps)
New Player "Housing":
Warships - customize your warship, recruit a crew, build your warship the way you want. Dress the living quarters in fancy furniture and lavish surroundings, or get grungy and gritty with pirate accents and holes in the wall, or anything in between!
New Primary Profession:
Carpentry - this will highly coincide with warships. There will be many other uses of the carpentry profession outside warships, and it is primarily meant to be a gold sink profession. Crafting fine wood and brass things for the warship is expensive!
New Secondary Profession:
Wood Gathering - Gather wood from all over Azeroth. There are several different types of wood to gather, oak, cherry, hickory, etc. New trees will be popping up all over Azeroth that can be gathered, similar to the wood mill in Warlords. This is the primary resource for the Carpentry profession.
All gathering professions are now moved to Secondary Professions.
More details at Blizzcon.