Death Knights
All players with a level 55 character will be able to create one Death Knight per realm, per account. (Gamespy.com)
Blizzard's designers decided to start the class with a full complement of abilities right off the bat, figuring that any player that has advanced a character to level 55 would be able to learn to handle the new class with minimal difficulty. (Gamespy.com)
Death Knights will have a summonable Skeletal Deatcharger Mount. (Gamespy.com)
Death Knights aren't using Mana or Energy, the runes are their primary source of power and are slotted into their “blade,” which lies underneath their health bar as a visual indicator of what resources are available to the player at any given moment. (Gamespy.com)
Spent runes automatically refresh after a set period of time, much like a Rogue's energy bar. (Gamespy.com)
There are six slots in the blade, and the player is free to fill those slots with any given number of Blood, Unholy or Frost runes. (Gamespy.com)
Death Knights also have a secondary resource called Runic Power generated by using your rune abilities. It will use the same mechanics as rage and increase when you deal damage, or slowly decrease if you're not using it. (Gamespy.com)
Death Knights will have a lot of abilities that cost all available Runic Power with varying levels of effectiveness based on total Runic Power spent. (Same mechanics as Execute for warriors, or finishing moves for Rogues) (Gamespy.com)
Death Knights will have spells equivalent to auras called “Presence”, providing them with powerful self-buffs depending on their role in the group. (One for tanking, one for damage dealing, etc …) (Gamespy.com)
An instant lifedrain spell (described as death coil without the fear component) will be available. (Gamespy.com)
Death Knights will be the first class to use diseases as a source of damage, with abilities that become strong depending on how many diseases are already applied to the target. (Gamespy.com)
“Army of the Dead” will spawn numerous undead minions to attack nearby hostiles targets. (Gamespy.com)
Death Knights will also revive an enemy (or ally) as a ghoul to use it as a pet with its own special abilities like a disease-inflicting strike and a stun. (Gamespy.com)
When an ally is risen as a ghoul, a pop-up will appear and this ally will have the choice to control the ghoul if he wants to. (Gamespy.com)
The Death Knight's particular niche will be in tanking magic-damage-dealing bosses. They will have an ability much like a banshee's anti-magic shell, greatly diminishing the amount of incoming magic damage they'll receive in combat (Gamespy.com)

Death Knight Skills
Death Coil – Depletes all Runic Power, dealing 600 damage to a non-Undead target, or healing 900 damage on a friendly target. (Curse.com)
Death Grip – This is the Death Knight's “taunt” ability. It also pulls the target to the Death Knight, forcing them to attack the Death Knight for a short amount of time. Yes, I said pulls the target; Blizzard is going to allow players to move mobs in the expansion both with Death Grip, and other knockback/pull abilities. This works on players too, so PvP balance ahoy! (Curse.com)
Chains of Ice – Roots the target in place. When the spell fades, it places a snare on the target that reduces in potency as the duration runs out. (Curse.com)
Death Pact – Sacrifices the raised ghouls to heal the Death Knight. (Curse.com)
Death and Decay – Targeted, AoE Damage-over-time which pulses similar to the Paladin spell Consecration. Anyone affected by Death and Decay has a chance to be feared. (Curse.com)
Frost Presence – Increases Armor by 45% and allows the Death Knight to generate 25% more threat. Only one presence can be active at any time. (Curse.com)
Unholy Presence – Increases Attack Speed and Movement speed by 15%. Only one presence can be active at any time. This was described by Tom Chilton as the “PvP” presence. (Curse.com)
Anti-magic Shield – Reduces the damage of the next magical spell cast on the Death Knight by 75%. It also converts the damage reduced into Runic Power. (Curse.com)
Strangulate – Depletes all Runic Power, dealing minor damage and silencing the target for up to 5 seconds. (Curse.com)
Summon Deathcharger – Allows the Death Knight to summon a Deathcharger mount. This mount is acquired through quests, similar to the Paladin and Warlock land mounts. (Curse.com)
Raise Dead – Raises a nearby corpse to fight for the Death Knight for 2 minutes. If used on a player corpse, the player has the option to play as the ghoul for the duration – gaining access to the ghoul's abilities. (Curse.com)
The ghoul has the ability to do the following:
Leap to the target
Rend for decent damage-over-time
Stun target, and of course more

Raids & Dungeons
All raid dungeons in Wrath of the Lich King will have both 10-person and 25-person versions. So at level 80, every major encounter and every big boss fight can be experienced with a group of 10. (Gamespy.com)
25-person raiding progression is not dependent on 10-person raiding progression. (Gamespy.com)
There will be no attunements or keys to obtain, and you won't be locked out of a 25-person instance if you decide to attempt the 10-person version, and vice-versa. (Gamespy.com)
25-person raids will earn more, higher-level rewards, in what should be the equivalent of one “tier” of loot quality. (Gamespy.com)
A new instance focused on the different dragonflights “The Chamber of Aspects” might be added to the game, but this isn't confirmed yet. (Gamespy.com)
Zero raid bosses have been designed at this point. (Gamespy.com)

Others
Players will unlock their flying mount at level 77. (Gamespy.com)
You can use your Outland flying mount, and new flying mounts will be available. (Gamespy.com)