Wrath of the Lich King
Level Cap: 80 (New talent (up to 51 points tier) will be added to every class.)
Skill Cap: 450
Continent: Northrend
Number of Zones: 10 (tentatively), for levels 68 and above
Profession: Inscription
Class: Death Knight (first-ever Hero Class)
Features: Outdoor PVP zone with siege weapons and destructible buildings.
Beta: Only closed beta announced for the moment
Release date: Unknown, but probably beginning of 2008.

Leveling

Leveling from 70 to 80 should take the same amount of time as 60 to 70. Don't expect this to go too fast because Blizzard won't let you use your flying mount in Northrend at the beginning. They're planning to keep you grounded until level 75 or 78 before you can use your uber netherdrake again.

Inscription

Inscription will be the new profession of the expansion. It will let you customize spells and abilities, for example you could make your fireball crit more often, or change the casting time of a healing spell.
You will also be able to create a totally new type of items, but there isn't any particular information about them yet.

Hero Classes

The hero class will be created on a new character slot once you completed the quest, you won't lose your original character.
The Death Knight should be the only hero class of the expansion, the devs want to see how the Death Knight works out before talking about new Hero Classes.

Death Knight

This is a tanking/DPS class, you will be able to tank with a 2H or duel wielding
Any race can become a Death Knight
You unlock the ability to create the Death Knight when finishing the tough quest chain, your character will not be overwritten, it will be retained (so you are not losing something to gain something)
Death Knight's will spawn into the world at a high level, they are still deciding which level you will start out at, 55,60, and level 70 have been discussed as possible starting points
You will not be Kill on Sight from your own faction, so you will still be able to browse the AH, there will be flavor differences with NPCs however, expressing their distrust of Death Knights
You get to choose your own type of Runeblade. You can choose from a traditional sword Runeblade or an axe variant
Death Knight abilities are dictated by 3 runes : Blood, Frost, and Unholy.
Your abilities will depend on how many of each rune you have. From what I saw from the demonstration, their are 6 slots for runes, so a balanced build would be 2 blood, 2 frost and 2 unholy. Spells will work like you use a spell that requires 1 blood rune, that blood rune is now on cooldown. Some spells will require like 4 of a rune for that ability to be used.
There will be talents to reduce the cooldown on runes (Talent trees are Blood, Frost and Unholy)
You will be able to summon minions but it is not the focal point of the class, it is just an accent to it, kind of like how Frost mages deploy their water elementals, summoning minions will not be integral to how you play the Death Knight
You will wear plate armor
The armor is said to be bulky and threatening like Arthas's armor. It will not look exactly like his. There were several different armor variations shown during the presentation that were definitely showing Death Knight look but also not looking like exact copies of Arthas's gear.

Known Death Knight skills

Unholy Embrace
Instant Cast. 5 min cooldown.
Target is embraced by a silhouette of darkness for 6 seconds, corrupting any healing spells and effects cast upon or currently affecting the target to cause damage for 50% of their healing potential instead of restoring health. Any damage shields cast upon or currently affecting the target will also be instantly consumed, dealing 50% of their absorption potential in damage. Once afflicted with Unholy Embrace, the target cannot be afflicted with Unholy Embrace again for 1 minute.
Army of the Dead
Raises several temporary minions (ghouls)

Zones
Having learned a valuable lesson about funneling 8.5 million players into once single starting zone with The Burning Crusade, Wrath of the Lich King lets players pick a zone on either side of the continent to start from: Borean Tundra in the west or Howling Fjord in the east. Each zone has level-equivalent quests (appropriate for approximately level 68 through 72) and one Hellfire Peninsula-style instance hub with multiple dungeon wings for level 70 characters, level 80 characters, and raiders. While Northrend immediately calls to mind ice and snow, Blizzard doesn't want to make those themes dominate Northrend completely-expect as much variety as ever, only not quite as weird and wild as Outland.

Dalaran (Capital)
Dalaran will be the capital city of Northrend. It will hover above the continent and you will be able to access it without even a flying mount. (You won't have it for your first levels in Northrend)
Basically the Blue Dragonflight has taken a turn for the worse and the Kirin Tor are concerned about it. What's about to ensue is a battle over magic and once Dalaran is rebuilt, it will be the staging grounds for that battle.
Dalaran on WoWWiki.com

Dragonblight
Graveyard of Dragons from the Blue Dragonflight.
Dragonblight on WoWWiki.com
Dragonblight Video on Youtube

Howling Fjord
This is one of the two entry points of Northrend, and is the zone where the forces of Arthas originally landed on the continent. In this region you will find the outpost of Valgarde, the Forsaken settlement of New Agamand, and a few other towns.
Howling Fjord on Official Site
Howling Fjord on WoWWiki.com
Howling Fjord video on Youtube

Utgarde Keep
This is the first dungeon appearing in WotLK. It's inhabited by Vrykuls (viking-like race) and should feature 4 wings once complete.
Utgarde Keep preview on WoWinsider.com
Utgarde Keep on WoWWiki.com

Borean Tundra
This is the second entry point of Northrend for players. This is where the horde has established the settlement of Warsong Hold under the command of Garrosh Hellscream (currently in Nagrand). You will also meet friendly Tuskarrs here.
Borean Tundra on WoWWiki.com
Borean Tundra video on Youtube

Grizzly Hills
Zone of the ... Furbolgs !
Grizzly Hills
Grizzly Hills video on Youtube

Other regions
There are still 6 unknown regions, but we can assume that they are Azjol-Nerub, Crystalsong Forest, Icecrown Glacier, Sholazar Basin, Storm Peaks, and Zul'drak

New races
No new races for players, but there are a few new NPC races. The most active one in Northrend will probably be the Nerubians, Tuskarrs, Vrykul, and of course the Blue Dragonflight

PvP
Lake Wintergrasp

Lake Wintergrasp will be an outdoor non-instanced PvP zone in the middle of Northrend. The main focus will be interesting reward affecting the whole region (or continent) and of course it will features ... Siege weapons and descrutible buildings !
The region is entirely avoidable and siege weapons won't be useable outside the designated area.

Siege Weapons and destructible buildings

Siege Weapons will be available in Lake Wintergrasp, and they're considering including them in old battlegrounds too. The current plan is to have mobile siege weapons so they can be positioned and aimed. For the moment we've heard about very "warcrafty" siege weapons, Meat Wagons, Ballistas, Catapults, Steam tanks etc ...

New battleground

A new battleground is also under development, it will have siege vehicles, 15 people in each team, and games will last 30~40 minutes.

Arena Changes

Arena weapons will probably have a rating requirement in the future, for example you'll need 1500 points to buy a weapon, but won't be able to buy it if your rating is below 1750 (this is an example, there isn't any value for the moment)
They might add a "Skirmish mode" to let you chose which team you want to face in Arena.
Arena Season 3 is scheduled, with a new arena and new gear.
Season 1 gear will be available through honor system when the season 3 starts.

Battleground changes

In AV, players won't spawn in the cave until there's nowhere else to spawn. Players will have the ability to report other players for being AFK, like the spam reports. You can remove this debuff by going in combat. If you don't, you won't gain any honor.
Honor gain will be more important at the end of a BG to make people stay.
Guild battlegrounds might be added (let guilds challenge each other, join battlegroup as a guild, etc). The ability to join AV as a group will probably be reactivated.
More items will be added to battleground's marks rewards.
They're also thinking about letting people chose the size of a battleground (15vs15, 20vs20, etc)

Dungeons

Wrath of the Lich King will have new 10 man and 25 man raids.
Arthas will be the final boss of one of the dungeon (but probably won't be available at the beginning because he's supposed to be the final boss of the expansion, expect a patch like Illidan)

Zul'aman
Nothing really new about the instance itself, except a few screenshots, but we had a nice preview of three loots here : [Dagger of Bad Mojo]
Dagger of Bad Mojo
Binds when picked up
One-HandDagger
142 - 213 DamageSpeed 1.80
(98.6 damage per second)
+21 Agility
Requires Level 70
Equip: Increases attack power by 40.
Equip: Your attacks ignore 154 of your opponent's armor.



, [Jin'rohk, The Great Apocalypse]
Jin'rohk, The Great Apocalypse
Binds when picked up
Two-HandSword
380 - 570 DamageSpeed 3.70
(128.4 damage per second)
+49 Stamina
Requires Level 70
Equip: Improves haste rating by 45.
Equip: Increases attack power by 120.


, [Formula: Enchant Weapon - Executioner]
Formula: Enchant Weapon - Executioner
Soulbound
Requires Enchanting (375)
Use: Learn how to permanently enchant a Melee Weapon to occasionally ignore 840 of your enemy's armor. Requires a level 35 or higher item.


.

New 25 man raid dungeon
A 25 man dungeon is scheduled for release after Zul'aman (and before the expansion). Loots will be better than tier 6 and it will have 6 bosses.

Caverns of Time
A new CoT instance will be added in WotLK.

Stats

Spirit will be improved in WotLK.
"Free" spell damage will be given to healers.

Classes
Druids

Mana issues are being looked at. (Moonkins)
Cat form's range will be tuned

Mages

Water Fountain and Muffin Factory (like the soulwell) are incoming. (No more "WATER PLZ !!!")

Hunters

A legendary ranged weapon will be available soon. (New 25 Man raid ?)
Mana issues are being looked at.
Many changes for hunters will be implemented in 2.3, more utilities for arena battles and group utility are planned.
Hunter's arena weapons prices might be lowered to reflect the fact that they need to buy two weapons (or even three).

Shamans

It seems that shaman talents were changed in the game version available during the Blizzcon, you can check them here

Crowd Control spells might be available in levels 71 to 80.
Totems are being looked at, The Mana Spring totem will be a lot stronger than it is and some other will be made stronger too.

Priests

Fear Ward will be spread around.
Lightwell and other abilities are being looked at.

Paladins

Stat distribution will be changed for retribution paladins, spell damage won't be as much needed as before.
The Season 3 Paladin Retribution set will have resilience.

Rogues

Rogue mobility in arena could be changed.

Warriors

They might up the damage a bit while tanking to make the class more fun.

Professions

Daily quests are coming in 2.2 and 2.3 (Cooking !)
More best-in-slot equipment will be added. (Give each profession a very good BoP item)
They want to make the leveling of crafts easier, especially from 275 to 300 (because it's not the "top" anymore)
Crafters should have the possibility to create great leveling gear (including 70 to 80 leveling equipment in Northrend to reward people who actually takes time to level their craft while they xp)
Itemizations gaps will be filled with crafting.
Vendor recipes will be more random and less campable

Engineering

Engineering is gonna get some love ... And more precisely, engineers will be able to craft a flying machine.

Alchemy

Transmutation will be changed, a daily cooldown will replace the current 24h cooldown. (Like instance resets)
Discoveries will be changed to make some of them easier to get. In the first place they were supposed to be "bonus", not something that you really need which is uber-hard to find. (For example, deviate delight would have been good as a discovery)

Jewelcrafting

A few epic gems were shown and presented as Zul'aman drops, you probably already saw them because they were all parts of the gems available in 2.2 PTR Patch.


Inscription
See first part of the news.

Cooking

A lot of new recipes will be available in Zul'aman, including melee hit food, spell crit food, a food that deals AOE damage to enemies near you, and a few other things.

Items/quest (anything I couldn't add in other categories)

Green items in Northrend will be better than current Tier 4 equipment.
Next patch (2.2 ? 2.3 ?) will add a dungeon daily quest, a battleground daily quest, and a cooking daily quest.
A lot of heroic badges loots will be added to the game with Zul'aman

Engineering

Engineering is gonna get some love ... And more precisely, engineers will be able to craft a flying machine.

A lot of heroic badges loots will be added to the game with Zul'aman