The Death Knight starting area will be an extended area in Northern Plaguelands. The developers feel strongly about keeping people interested in playing in the Old World so they are creating this new zone. I personally hate every zone that has the word "plague" associated with it. Admittedly, I didn't reach either of the current Plaguelands until after I'd upgraded to The Burning Crusade, which meant I really had no time for grinding there since Outland awaited alluringly. However, it will be interesting to see if they make changes to the existing Western and Eastern Plaguelands along with adding content to the North to make these areas more interesting to players -- be they Death Knights or not.
Death Knights can be any race. As one speaker said, "The Lich King doesn't discriminate." (I just knew he was a standup guy!) The specific example they gave is the concept of playing a Gnome Death Knight. Man, I cannot wait to see a Gnome Death Knight! I mean, it boggles the mind. Along those same lines, the voice emotes will be changed for Death Knights according to race. A Tauren DK will sound very different from a Dwarf DK; conversely, you will also be able to tell the race of the Death Knight by its voice. For me, this gives me added incentive to play races I've never tried like Orcs or Dwarves, just to see how they might differ, even if it's just for a few laughs.
We knew previously that DKs use runes for their abilities. Today we learned that they will have a 10 second cooldown. The short cooldown is meant to illustrate how the Lich King is able to command literal Armies of Undead. This prospect sounds awesome. I seriously want armies of undead at my disposal, like a wall of rotting flesh hurling itself at my enemies! I wonder how tough these dudes will be?
In terms of what role the Death Knights will play in battle, the developers said they didn't want to punish the player based on what spec they choose. Even though it's problematic for designers, they are sensitive to WoW's tank shortage problem. They want to avoid forcing DKs to choose between tank spec or DPS spec. They are searching for a more flexible solution to the problem, giving a player as many options as possible. For example, during raiding, they want tanks to be interchangeable for the 5-man level, but they should have a specialty at the 25-man level. Although they envision Death Knights being the best tanks versus casters, they should also play decently well against other foes.
In general, the developers have said that they want to make switching between specs easier for every class, not just Death Knights. I think this makes it much more appealing to try out tanking (which I've never done) because then it doesn't seem like a grave mistake if you end up being horrible at it or not liking it. The flexibility of the DK class is going to be a huge draw and it's going to make it very fun to play.
For more info, check out WoW Insider's Death Knight guides.