Since Mage is my main class, I've decided to write about forthcoming changes and how these affect your usual pre-Legion rotation, stat priority and more.
Arcane Mage Legion Changes
Mastery has been overhauled, meaning damage will not scale depending on your current Mana level. Plenty of glyphs have been removed from the game entirely and a new set of passives and abilities has been added. Currently Arcane Mages lack sustained damage which is vital during boss encounters. They are great for ordinary dungeons, but the other two specs are simply better.
Arcane Mage +++
Arcane Mages have great single-target burst damage, a good damage output on stacked targets, they aren't cooldown-reliant, have excellent mobility. They are awesome when assigned a specific task, such as kiting, add nuking. Greater Invisibility grants them defense and their damage scales up well with gear.
Arcane Mage ---
Little to no sustained damage dealt to single-targets, low cleave damage and the reliability on cooldowns for single-target damage is what is disadvantegous about Arcane.
Your Primary resources include Mana and Arcane Charges. The charges are generated by basic spells and certain talents. You can have a maximum of four stacks. As they increase your spell Mana cost by 100% per stack, you need to be careful about your Mana management. The stacks also increase your damage output by a % (based on the amount of your Mastery). Mana management is crucial. Even though you deal Maximum damage with four stacks of Arcane Charges, keep in mind that your Mana will deplete fast and you will end up standing there waiting for it to regenerate which is how you shouldn't be playing Arcane.
Your main ability is Arcane Blast. It generates a single Arcane Charge whenever cast. It has almost no Mana cost initially, but upon building stacks, you will see your Mana disappear within a few casts. With four stacks it has no travel time, so it's a great spell for finishing off priority targets during encounters.
Arcane Missiles is your next ability, which is a channeled spell and deals more damage than Arcane Blast. It's only available when it procs from other spells. It also generates one Arcane Charge. Your damaging spells have a 15% chance to generate a proc of Arcane Missiles and Arcane Blast has a 30% generation chance. Up to 3 charges can be stored in total. The fact that it costs no Mana makes it a must-use spell when you're at four stacks of Arcane Charge. It enables you to bypass the Mana depletion multiplier while still taking advantage of the increased damage output. The spell has a limited travel time, which is why you should not use it to finish off targets, since it's likely they will be dead by the time you finish your channel. Use the ability whenever you have three stacks so you don't overcap.
Arcane Barrage allows you to get rid of your Arcane Charges. Understand the ability as a Mana maintenance tool, it helps you manage your Mana a lot. So essentially, generate charges with Arcane Blast taking advantage of the damage buff with Arcane Missiles and be rid of your charges with Arcane Barrage.
1. Arcane Power is your main DPS tool. Your spell damage is increased by 30% and the Mana cost of spells is reduced by the same amount. Despite its short duration it allows a potent burst potential. Best combined with 4 Arcane Charges and a decent amount of remaining Mana.
2. Evocation is not your damaging spell, you will need it to regenerate depleted Mana that's why it should be cast right after Arcane Power's over, because that's where you need to get your Mana back. Since Mana means more damage this spell has a high value.
3. Time Warp increases Haste of you and your party by 30% for 40 Seconds. This is basically Heroism, so usage varies. Use it whenever you have troubles finishing your target before he finishes you. No really, just use it when your Raid Leader announces it in a raid environment. Note this ability puts a debuff on you and for the next 10 minutes, you will be unable to benefit from it (shares debuff with Heroism, Bloodlust, Netherwinds and similar spells).
Arcane Mage Stat Priority
As an Arcane Mage, you should focus on the following stats to maximize your damage:
Intellect > Mastery > Crit > Versatility > Haste
All your gear should have Intellect as it's your primary attribute. It gives spell power, which increases your damage. Arcane Mage has a mastery which increases your mana pool and regeneration, enabling more spell casts at maximum charges between Evocations. It also increases the damage gain per charge making it an unmatched stat. Haste increases your spell cast speed and also reduces your cooldowns. Ciritcal Strike increases the chance to critically hit with spells and abilities, while Versatility is a damage done increase / damage taken reduction.
1. General Usage - use this build during leveling, dungeons or questing.
2. Raiding - this build should be used whenever you find yourself in a raiding environment.
Ability Usage Phases
The point of the whole rotation is proper Mana management. The key is stacking up your Arcane Charges while maintaining a stable Mana pool, a so-called Conservation phase where you build up your Arcane Charges and next is a Burn phase followed by a Mana Regen phase via Evocation to get your Mana back.
The goal during this phase is to save enough Mana for your burn phase to still last for all of Arcane Power while dealing enough damage to be efficient. The more Mastery you have the more 4 Arcane Charge casts will you be able to manage without draining the entirety of your Mana Pool.
During this phase, use Rune of Power if it has two charges. Cast Arcane Orb and you don't have any Arcane Charges. Apply Nether Tempest (if talented) with 4 stacks of Arcane Charge. Refresh it with 4 stacks of Arcane Charge if it has less than 4 secs remaining; don't forget to apply it if it's missing. Cast Arcane Missiles if you have 3 charges of Arcane Missiles or if you're not at 100% Mana and you have 2 or more Arcane Charges (the more charges the better). Cast Supernova. Cast Arcane Barrage if you need to regen Mana. Cast Arcane Explosion when you find yourself fighting 3 or more targets. Again cast Arcane Blast.
This phase begins at the start of your encounter or whenever Arcane Power is off cooldown. It's important to note that you will not be interrupted during burn phase, so avoid entering it during heavy moments or when destroying boss mechanics are up.
The Main Goal
The main goal during this phase is damage maximization during Arcane Power. This spell and Evocation both share a 90 second cooldown, so it's essential to burn all your remainder Mana during this phase. Once Arcane Power ends, and you don't have enough Mana to cast Arcane Barrage, you should activate Evocation do a full channel and restore your Mana to max. After this is done, you're back in conserve phase. Arcane Missiles should be saved for after Evocation if you reach 0 Mana.
Make sure you have four Arcane Charges. Place Rune of Power in a a safe spot. Activate Arcane Power and cast Arcane Missiles whenever available. Cast Arcane Blast and activate Presence of Mind for the end of Arcane Power. If there's AoE or cleaves and you've got Resonance talent, it's the ideal time to cast Arcane Barrage at the end of Arcane Power for increased cleave damage. If you talent into Charged Up, feel free to cast Arcane Barrage after Arcane Power ends for extra damage, then use Charged Up and continue burning your your foes.