Death Knight
World of WarCraft's first Hero Class is a plate-wearing tank/DPS hybrid that works a little something like this: When players hit level 80, they'll be able to embark on a quest (similar in difficulty to the Warlock's epic mount quest, back before the level cap was raised to 70) that unlocks the ability to create a Death Knight character. The Death Knight starts at a high level (somewhere around 60 or 70, though Blizzard isn't certain yet), so you won't have to grind your way back up all over again. It's intended as an alternative, advanced class for end-game use only.
Rather than using rage, energy, or mana, Death Knights have a special "rune sword" displayed beneath their health bar, onto which the player can etch six different runes (choose between Frost, Blood, and Unholy). Different abilities require different mixes of runes, and using abilities consumes the requisite runes until a cooldown timer causes them to be available again. Stuff like talents, spells, and other specifics are still being finalized. So are key questions like what races can be a Death Knight (start a Gnome petition now) and whether unlocking the Hero Class on one server means you can create a Death Knight on any server.
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.
Inscription
Wrath of the Lich King will introduce a new profession called Inscription that will let players customize individual spells and abilities in their spell book -- to change the cooldown timer on a Fireball, for example, or alter the crit rate of a healing spell.
Outdoor PvP
One of Northrend's 10 new zones (final count still tentative) is an outdoor PVP zone that introduces the concept of siege weapons (Meat Wagons, Steam Tanks, etc.) and destructible buildings. Blizzard describes the outdoor zone as basically a "non-instanced Battleground" where completing objectives earns world-wide bonuses for your side (Think Spirit Shard buff in Auchindoun instances). The PVP zone is entirely avoidable (if you're the carebear type), and seige weapons won't be useable outside their designated PVP area.
Northrend
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.
Northrend's indigenous inhabitants include a Viking-like race called the Vrykul: 15-foot-tall humanoids with a strong Norse influence. Most Vrykul you encounter will be your enemies, as they've been in Northrend so long that the Lich King holds a lot of sway over them. Another friendly race -- the walrus-like Tuskarr -- will serve as a cute little counterpoint to the lumbering undead-loving oafs.[/b]