Worldofwar.net published a big overview of Blizzard's presentation on their upcoming World of Warcraft: The Wrath of the Lich King expansion. Here's a look at the main headlines:
All 25 man raids in Wrath will also be available to complete with 10 players. This is so that more players will get to see end game content (Even the Arthas encounter will be available as a 10 man raid).
There will be completely different loot tables for the two different raid types. They will also be on separate cool downs, so players could fight in both the 10 man and 25 man raid on the same day if they wish.
The mounts available to buy in Wrath will probably be able to take passengers. These mounts will also work in the old world (Provided they are land mounts), so players can give low-level characters a ride! The new mounts will also be slightly faster, around the speed of the Armored Netherdrakes.
All 5-man instances in Northrend will feature Heroic modes, and the loot tables for the 2 modes will be totally separate. Heroic dungeons will have their own "Tier" of armor completely unique to that level dungeon.
Death Knights will be available to players who have a character level 55+ on the same realm. There is no other requirement at this time.
Players are allowed 1 Death Knight per realm that they have a level 55+ character on.
Death Knights will be "born" in a floating Necropolis over the Eastern Plaguelands, similar to Naxxramas.
Death Knights will start at level 55.
A Death Knight's starting armor will be a mixture of green and blue quality items. The first few quests that they encounter will provide upgrades to these starting items, and these upgrades will be armor specific to Death Knights.
Death Knights can dual wield.
One Death Knight spell is Raise Dead, which converts a fallen unit into a ghoul for a time to serve the Death Knight, similar to a Hunter's pet. If the spell is used on a fallen friendly player, the player will be presented with an option box similar to a resurrection message, and if they accept, they will take control of the ghoul and will be able to use its abilities for the period of time that it is active. This can work as a sort-of battle res, and Chilton stated it may be available in Arenas.
Northrend is Huge. It is larger than Outland by a couple of zones, and some of the zones in Northrend are bigger than the largest zone in Outland.