As with any expansion, a lot of changes have rolled out in patch 7.0.3, the Legion expansion pre-patch.  For full comprehensive patch notes, you should check out the full patch notes here.  For the quick and dirty summary, here is what you need to know:

Classes
Note: There were also a number of class changes as part of the overhaul, but for those you’ll want to read the official patch notes to get all the details on your particular class. 

• Gnomes can now be hunters.
• The hunter Survival spec is now a melee spec
• The rogue Combat spec was removed and replaced with Outlaw, which is pirate-esque thematically.
• Demonology walocks lost metamorphism and have many new demon-centric spells.

Talents, Specs, & Glyphs

• Specs and talents have been totally reworked, and a lot of spells removed or limited to particular specs.
• Major glyphs were removed.
• Minor glyphs are now applied directly to the spell in your spellbook.
• You can now change your spec or individual talents on the fly.  Changing is free when in a rested area like a city, or can be done with a scribe-crafted reagent when out in the world or in an instance.

Buffs & Stats

• Raid buffs are gone.
• Multistrike was removed.  It was replaced with crit on existing gear.
• Spell power was removed.  It was replaced with intell on existing gear.
• Spirit and Bonus Armor were removed.  Both were replaced with versatility on existing gear.

Gear

• Legacy (pre-Warlords) tier set bonuses no longer work.
• Plate-wearers now start in plate from level 1 (previously mail until 40).
• Mail-wearers now start in mail from level 1 (previously leather until 40).
• Legendary ring chain is now unavailable to pick up fresh.

Transmog

• Transmog gear is now account-wide, with some restrictions.  See this video for details.
• You can now hide your shoulders!
• Hat, cloak, and shoulders are turned off at the transmog NPC rather than in the Interface settings.

Toys

• A ton of things were added the toybox. This includes many engineering items (teleport items, mailbox, blingtron, etc) but they still retain a profession restriction.

PvP

• Honor and Conquest points are gone.
• Gear is now purchased with Marks of Honor which are obtained in BGs, arenas, etc.  Old school PvPers might remember a similar type currency long ago!
• PvP gear is now normalised when in a PvP instance.

General Changes

• All regular mobs are shared tap with up to five players now.
• You now have 12 character slots per server, to make room for the incoming Demon Hunter class
• The gold cap has been increased to 9,999,999 gold.
• Garrison gold missions removed.

User Interface

• Character stat pane has been pared down and now only displays stats relevant to your character.
• New optional resource bar can be displayed under your character
• Max zoom script disabled :sadtrombone:

Now that you’re ready to jump back in the game – renew that sub, dust off your character, and prepare for Legion!