Park Kraken:
You become a Death Knight after you reach lvl 80. It's sort of like how when you reach a high enough lvl in your profession, like Blacksmithing, when you can choose what to specialize in.
From what I've gathered by reading around, Death Knight will be able to raise from the dead anything that he kills, and can have more than one "undead pet" at his at any given time.
Well that's just speculation, I did manage to find this nugget though...
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.
I find this VERY interesting. I can't wait until more details are released on this. The new professions sound pretty cool too.
BTW Hyfe the level cap is going to be 80 now. Seems way too soon though imo. There aren't that many guilds which are progressing very quickly through the current end game content.
I don't know how this is going to affect current endgame raids such as Hyjal/BT etc. which many guilds have yet to venture into. If the expansion comes out and all of a sudden level 72/73 greens are equivalent to level 70 raid epics (which was the case when TBC came out, level 60 epics were quickly replaced through green quest items).
So what I'm wondering is how this will affect the current raids that many guilds have yet to experience, i.e. Hyjal. When TBC came out most guilds had never gone into Naxx, and many of the ones that had never got very far. Once TBC came out nobody cared, I can count one time that a guild went back into Naxx after the expansion and I know my guild did a 10 man MC after the expansion.
Who knows, I can't wait until more information comes out though.