OKAY HERE WE GO
You can find the detailed changes on Wowhead here. If anything below is marked with a (?) then it's likely something that needs confirmation/clarification.
- Every class will gain four new traits plus a new paragon trait, Concordance of the Legion, which is a minimal primary stat buff proc. A tl;dr of the traits is "dps upgrade" for dps, "healing upgrade" for healers, etc.
- The new traits consist of a 1/1 which provides the bonus of the old 20/20 paragon, a minor 4/4 trait, a 1/1 major trait, and a 1/1 golden dragon trait.
- Trait costs spike drastically after the 36th trait (1.9M to 10M), so make sure you grab the first new trait for a free 10% damage/healing/armor before anything else
- In addition to the new traits, every existing minor trait (those which have 3 ranks on live) can now go up to 4 ranks.
Dlsclaimer: Blizzard has said they do not want to do massive class balancing the middle of a tier, so a lot of the changes are minimal (if any at all). The biggest changes will be the new traits, which will not be commented on here (as there are over 100 of them and I am not widespread enough of a player to know the impact of all of them). See the Wowhead link above for those details.
- Blood DKs generate more threat with grip (200% live vs. 400% ptr)
- (PVP) Frost DKs do less damage with Chill Streak
- Unholy DKs are abandoned like usual
- Vengeance's Demonic Wards focuses more on damage reduction instead of armor (woo magic resistance!)
- Havoc is left alone
- Balance's Starfall no longer scales with haste (RIP)
- Feral is abandoned like usual
- Guardian deals more damage with Mangle/Thrash, less with Swipe.
- Restoration is left alone
- (PVP) Binding Shot is less effective on players
- Beast Mastery's Stomp (Dire Beasts) deals slightly more damage
- (PVP) Marksmanship's Scatter Shot now incapacitates instead of disorients
- Marksmanship's Deadly Aim talent gives now gives half of the origin benefit
- Survival is left alone
- Arcane's Mark of Aluneth now deals increased damage and pulses to every enemy within 6 yards.
- Fire's Blast Wave deals moderately more damage and Cauterizing Blink now heals for twice as much as before
- Frost is left alone
- Brewmaster is left alone
- Mistweaver gains Fortifying Brew as a baseline ability (max hp and damage reduction by 20%). Mistwalk has been removed and replaced with Chi Wave.
- Windwalker is left alone
- Judgment of Light will now stack between all of the specializations. No need for the tank to change talents if the holy paladin is using it now.
- Holy is left alone
- Protection's Retribution aura will no longer reset your autoattack swing timer (bugfix)
- Retribution is left alone
- Discipline is left alone
- (PVP) Holy's Spirit of the Redeemer talent now resets the cooldown upon death.
- Shadow's Mental Fortitude can only shield up to half the original amount. (8% max hp live vs. 4% max hp ptr)
- Assassination's Bag of Tricks now has a slightly lower chance per combo point (3% live vs. 2.5% ptr) to activate but can now activate off of Rupture instead of just Envenom. Urge to Kill now grants 60 energy plus 60 over 2 seconds instead of instantly filling your energy bar.
- Outlaw now deals slightly more damage (3.5%) in multiple abilities. Curse of the Dreadblades now procs off of Ghostly Strike as well. Fatebringer nerfed to 1 reduction.
- (PVP) Plunder Armor now focuses more health reduction and less on damage dealt reduction.
- Subtlety is left alone
- Elemental's Earth Elemental now lasts until the totem is destroyed (?). Storm Elemental now has a 30 yard range.
- Enhancement is left alone
- Restoration is left alone
- Affliction's Reap Souls has received some significant changes. It will no longer reset the duration of the current buff, it will stack (up to 60 seconds). It also now has a 5 second cooldown. Lastly, souls are reset to 0 at the start of an encounter (note: start of M+ or raid fight, not per dungeon boss)
- Demonology is left alone
- (PVP) Destruction's Fel Fissue now lasts significantly less time.
- Arms is left alone
- Fury is left alone
- Protection's Intercept now roots the target if cast on an enemy (?).