Now that we've had blogs on all the classes that are currently playable - the ones that you've already played, in other words, unless you were at Blizzcon and got to take the Demon Hunter for a spin - I figured I'd make a quick(ish) rundown of the various classes and specs and how they're changing. This will be short and sweet, so if you want to get more information, look at the individual class blog posts (Druids and Monks were combined in one post just because I read both blogs around the same time.)
The main baseline change they kind of slipped in there is that Focus is apparently going to have a baseline regeneration of 10 per second, up from 6, which I think makes it identical to Energy. I'll keep an eye on this.
Beast Mastery is going to focus a lot more on summoning pets, using Dire Beast as its main Focus regenerator.
Marksmanship will be focused on marking targets (with Hunter's Mark) for future pain.
Survival is now a melee spec, striking targets with vicious successive Mongoose Bites while weaving in other attacks that use Focus.
Mages are probably the least changed class, to be honest, still maintaining a lot of the familiar functionality.
Arcane will now get Arcane Power as a secondary resource, and is no longer penalized for burning through a lot of mana to get a burst of damage, thanks to a new mastery. There's still the old risk of going OOM from spamming Arcane Blast, but your damage won't drop until you can't cast spells anymore.
Fire is getting minor tweaks, perhaps most profoundly getting a redesigned Combustion that works as a more typical damage cooldown. Ignite also now automatically spreads between targets, so Fire will always have some automatic cleave/AoE action.
Frost is working much the same, but Frozen Orb is becoming rotational, replacing Frostfire Bolt.
You can now kind of divide Priests between White, Grey, and Black sort of, with healing, a mix of healing and damage, and straight damage.
Holy is largely similar, though Chakras are going away (maybe that's big - I don't play Holy.)
Discipline is going all-in on Atonement healing, marking allies for Atonement and then healing them through attacking enemies.
Shadow has a brand-new resource that replaces Mana and Shadow Orbs, and is called Insanity. Shadow will be about building up Insanity and basically trying to stay Insane as long as possible to do lots of damage, though the sources of damage are pretty familiar.
The biggest deal is that Holy Power is now for Retribution only. All specs are getting some changes, though Holy probably most of all.
Holy will now be incentivized to really wade into battle, getting a new mastery that increases healing based on your proximity to the target and also some self-sacrificial spells to help allies.
Protection has many of the same abilities, but it has a dedicated self-heal based on missing health and Shield of the Righteous now is a three-charge ability with no resource cost. Getting charges of ShotR will be a mechanic. Also there's some funky AoE stuff that I wonder whether it'll make it live.
Retribution is getting re-oriented toward melee strikes as its major source of damage, and is the spec that retains Holy Power.
All Warlocks are going to be using Soul Shards again, but Soul Shards should be a more interesting resource.
Affliction is focusing more on DoTs, with Unstable Affliction being more of a burst-DoT that you spend your Soul Shards on.
Demonology is very different, focusing on summoning waves of demons and using spells to empower them.
Destruction has a lot of the same kit, burning targets, but a funky new mastery that increases damage randomly.
The huge change for DKs is that Runes are now all universal, though they still only regenerate 3 at a time. The other profound change is that each spec has its own, single disease.
Blood is once again getting a bit more focused on physical strikes, and Bone Shield is now more of a rotational effect of a new ability.
Frost is dual-wield only, of course, but it looks like they want to use more of the 2-hander rotation, with Killing Machine only affecting Obliterate. Fine with me! Also, Frost uses a redesigned Remorseless Winter instead of Death and Decay.
Unholy is focusing a bit more on disease effects and things related to diseases, with a new Festering Wound mechanic that adds a little unpredictability.
The huge change is that two of the specs are getting a different primary resource called Maelstrom.
Elemental will build up Maelstrom with their nukes and then spend it with Flame Shock and Earth Shock - which no longer have cooldowns.
Enhancement uses new short-range spells (kind of themed on old weapon imbues) to build up Maelstrom and then spend it on Lava Lash and Stormstrike.
Restoration seems mostly unchanged.
Warriors in general seem mechanically similar to how they've always been, but will obviously get some changes based on talents.
Arms now has a few more buttons to push, and the Cata/Mists version of Sudden Death (though it's now called Tactician) that lets you use Colossus Smash more often and less predictably.
Fury is getting somewhat less proc-reliant, with abilities you can use without guilt when you're not enraged, and changes to Raging Blow to allow you to use it as much as you want (if you have the Rage) while you're enraged. Also, Enrage grants an enormous haste buff, which will also help you generate Rage.
Protection is getting minor tweaks, mainly swapping out Shield Barrier for a kind of on-demand significant damage reduction ability called Ignore Pain.
Much like Paladins, the other tank/melee/healer hybrid, the tank and healing specs are losing a secondary resource, though this is arguably more profound for Monks, given that Monks have always used Chi (of course Paladins have had Holy Power longer so... oh well.)
Brewmasters will focus a lot more on brews, trading between Ironskin Brew and Purifying Brew (which share charges) to either raise Stagger % a lot or purge that damage. Breath of Fire should now be a more commonly used ability, acting as the AoE equivalent of Blackout Strike (not kick,) which are both pure threat moves.
Mistweavers are losing Fistweaving, focusing entirely on their heals and getting some new ways to passively channel heals if things are going smoothly.
Windwalkers are getting more of a focus on the fighting kind of gameplay, with more encouragement to mix up their attacks, as well as a way easier to use Storm, Earth and Fire.
Druids are going to have to do less shapeshifting to perform their other roles, with some talents that give nice other-spec features regardless of your form. Also, Balance and Guardian are getting some significant changes.
Balance has a new primary resource bar and no longer uses an Eclipse meter. Instead, their Solar and Lunar spells will build up Astral Power, which is then spent on Astral spells, which will then buff the Solar/Lunar ones.
Feral looks mostly unchanged.
Guardians will focus a lot more on high health, armor, and health regeneration and less on dodging attacks, though the offensive abilities are mostly the same.
Restoration is also largely unchanged.
Rogues aren't getting their fundamentals redesigned, but two specs are getting profound reworkings, so much that Combat isn't even Combat anymore, and is now Outlaw. Also, Poisons are now Assassination-only (as are all bleed effects.)
Assassination seems mostly unchanged, though they made no mention of Dispatch, so I wonder if that's getting removed.
Outlaw will have a mix of melee and short-range attacks, with Saber Slash (the new Sinister Strike) potentially making Pistol Shot free. Yes, Pistol Shot (you don't need to equip a gun.) Looks like it will be somewhat more unpredictable and definitely very different flavor-wise.
Subtlety is focusing a lot more on stealth as a mechanic, with some Shadow damage mixed in and a redesigned Shadow Dance, now a passive that drops you into stealth sometimes when you use finishers, allowing you to use a new opener, Shadowstrike, which is kind of Shadowstep+Ambush.