There are three factions in Aion: Elyos of Elysea, Asmodians of Asmodae, and Balaur of the Abyss. All three continents are contained in the world of Atreia, connected by the Tower of Eternity, and overlooked upon by Aion (aka God). The Elyos and Asmodians are playable races. The former is a bit lighter and the latter is darker, though neither side is truly evil. Balaur are the dark ones; an NPC race of dragon-like demons bent on the domination of Atreia.
Think of it as Alliance, Horde, and... the dragons of WoW (but way more evil). The difference between WoW and Aion is that the conflict between all three factions is much more in the forefront of your gaming experience. As an Elyos you will fight Asmodians and vice versa. You can try avoiding each other, but it will be extremely difficult. If PvP makes you queasy, Aion probably isn't the game for you. Both factions are aggressive to the Balaur and the Balaur are equally aggressive to both factions, though they may side with the weaker side from time to time.
You can only choose from four archetypes when you make a character (Warrior, Scout, Mage, and Priest). You play as this basic archetype until you reach level 10, at which point you branch off into a more specialized class. The classes of Aion should be familiar to anyone with experience playing WoW. They are:
- Templar (like Paladins) -- Great sword-and-board tanks but no healing
- Gladiator (like Warriors) -- More offensive semi-tanks that can use any weapon
- Assassin (like Rogues) -- Stealth, dual-wield, burst-DPS
- Ranger (like Hunters) -- No pets but traps and stealth
- Sorcerer (like Wizards) -- Nuke, CC, and nuke some more
- Spiritmaster (like Warlocks) -- Pet summons for all situations
- Cleric (like Priests) -- Like to heal and resurrect -- a lot
- Chanter (like Druids) -- No animal morphing but jack-of-all-trades
Combat is also quite similar between the two games. You have the holy trinity of damage-dealing, tanking, and healing all set in a medieval combat environment with melee, dodging, parrying, blocking buffing, debuffing, direct damaging, damage over timing, healing, etc. There is also quite a variety of crowd control in Aion in the form of stunning, sleeping, silencing, rooting, etc.
Templars tend to tank. Gladiators tend to melee AoE and off-tank. Assassins get in, take out a squishy, and get out fast. Rangers like to shoot you from afar and keep you at bay with traps. Sorcerors want to light you up. Spiritmasters would rather their pets light you up. Clerics are quite sturdy in their own right but are mostly team players. Chanters prefer to melee, heal, buff, and debuff.
Player vs Environment (PvE)
Aion has 50 levels. The only way to gain levels is to gain experience points. You gain XP in Aion by killing mobs, completing quests, and gathering resources. You'll kill mobs while questing, raiding, or simply camping a grind spot. Each time you gather a crafting resource you're also rewarded with XP.
The quests in Aion are run-of-the-mill: Kill ten (insert creature name here); Deliver this to that person; Go talk to him who will send you over there and then you report back to me. They don't really differ much from the quests you'll find in WoW. You have special campaign quests though, which provide a deeper storyline. These are usually associated with cinematic cut-scenes.
Aion has a number of raid instances as well, though not as many as you'll find in WoW. The early levels are especially barren of instances. You can find a complete list of instances here.