Right, first thanks to Blizzard Watch and MMO-Champion for their liveblogs.
The Systems panel covered a whole lot of information. I'll try to get the important bits here. Thankfully, it was pretty organized, so I'll divide it the way they did.
Using the Affliction artifact, the Deadwind Harvester, as an example, they showed how you recover it (a quest in Deadwind Pass that takes you underneath Karazhan.) The artifacts will actually have at least one (though maybe more?) active traits - for example, Deadwind Harvester will have an extra DOT called "Unleash the Harvest." There also seem to be some crazy passives, like souls that try to escape the staff that you get a damage buff for hitting.
Artifact Power is gained through myriad methods like quests, dungeons, raids, etc. This seems to be what unlocks the sort of talent-tree for the artifact over time - something they expect will take a long time. I believe they have said that your alt-spec artifacts will gain more artifact power based on how much higher your mainspec one has, so you can catch up decently, but having two maxed-out ones will take a whole lot of effort.
Artifacts will also get relics. It looks like you'll have a couple sockets to switch out relics in, and these will give basic bonuses like raising the item level, but also buff some of the Artifact Traits (for example, there's an Affliction one that increases the crit chance of Unstable Affliction, and you might get a Relic that increases that bonus in addition to buffing the weapon.
There are a ton of appearance options for each artifact, each with several color variations. It even looks like you'll be able to change the weapon type with some of them, like the Marksmanship Bow and the Beast Mastery Gun both seem to have a Crossbow variant. I wonder what this might mean for the Fury swords and whether you can switch them from 1H to 2H or vice versa.
You'll customize your artifact at a special shrine in the Class Hall.
Druid Class Hall will be the "Dreamgrove," which is an area in tune with the Emerald Dream (not sure where.)
Shaman hall will be the Eye of the Maelstrom.
Class Champions might be leaders of their own factions, but they'll have stuff for you to do. Examples were Lady Liadrin for Paladins and Risstyn Flamescowl, an Orc Warlock form the Council of the Black Harvest.
Class Halls will have special features for different classes, like pickpocketing missions for Rogues or special Demon Hunts for... Demon Hunters. There will be some kind of brawling arena in the Warrior hall where players can fight each other. They want the halls to reflect the breadth of each class, so for example, berserkers, (non-magical) knights, and old grizzled veterans should all feel at home at the Warrior hall.
Hunters will have a special network of Eagle flight paths. Druids will also be able to traverse the Dream to get around Broken Isles.
Everyone gets a Death Gate/Zen Pilgrimage-type spell so that they can come and go as they please.
Some of this was covered in my earlier post. Customization options are pretty darn cool, and DHs will even have a brand new VO - no generic Night Elf/Blood Elf voice. Presumably this means all-new /silly jokes?
Fury will be a glowing purple bar. It looks like it does not degrade out of combat. The DH interrupt instantly fills the bar, so it looks like it will be easy-come-easy-go.
Lots of big changes to resources, etc.
Arcane Mages: New mastery, and Arcane Charges are now a resource bar.
Death Knights: Only one type of rune - essentially all runes are Death Runes now. That's... pretty big. Not sure how to feel about it.
Fury Warriors: No more "charges" of abilities. Everything's basically there for you to use, though I don't really understand how this will work.
Subtlety Rogues: Really doubling down on the ninja aspect. Shadow Dance can now put you back into an unbreakable Stealth. Shadow Strike is a kind of mini-Shadowstep+damage. They got rid of a lot of the "generic" Rogue abilities (not sure what those are.)
Talents will no longer come in "theme" rows, which they hope will make the choices feel more meaningful. Lots more spec-specific talents, meaning hundreds of new designs.
Building up your professions will be much more of a story/adventure. For example, you'll have to research what kind of cloths they use in the Broken Isles and collect recipes.
Recipes will get ranks and get more impressive as you get higher ranks, and also possibly be easier to make, with fewer materials or shorter cooldowns (how about no cooldowns for professions? Anybody?)
Crafted items are meant to be more meaningful. For example, goggles this time around will be made out of gun parts and actually shoot bullets at enemies (Bullet Goggles!?!?) Lots of cool examples, like a necklace that summons a druid to help you in combat.
Unwanted crafted items can be composted into Obliterum, which will then be used to upgrade the item level of existing items.
Inscription: Major glyphs are gone, and Minor Glyphs will only be limited to one-per-spell. Inscription will now make special runes that increase effectiveness against a particular boss (presumably you'll get the one for whatever boss you're currently working on.)
Archaeology will be essentially an expansion-long adventure. Fishing will also have more meat to it (I'll believe it when I see it!)
World drops will be Legendary. Whoa, what?
Legendary world drops will have unique effects, sometimes specific to certain specs, like a pair of Val'kyr boots that causes Avenger's Shield to reduce the cooldown on Guardian of Ancient Kings.
If you get Personal Loot that's a downgrade from what you already have, you can give it to a party member.
Every item you've ever had, plus EVERY QUEST REWARD FROM EVERY QUEST YOU'VE EVER COMPLETED, REGARDLESS OF WHICH REWARD YOU CHOSE will be available to you.
You can save outfits and switch them at the click of a button. You can save "hidden" slots with this. Oh, and you can HIDE SHOULDERS now (no more football mages unless you want to be a football mage!)
Weapon Enchants, Tabards, and Shirts are now all part of that transmog system.
Holy crap, the bag space! The BAG SPACE!
And that's it. Class blogs are going up tomorrow, so if you want specifics on, say, your Shadow Priest's new tentacle-powers, you won't have to wait long.
Q&A's up next.