Vaneras posted a FAQ for the Death Knight class on the EU General Forums today. This FAQ is very informative and answers a number of questions regarding the first Hero class to be introduced to World of Warcraft.
There are a lot of things still being worked out with the Death Knight class such as specific spell abilities. Unlike a traditional caster type however, a Death Knight's spell power comes from runes that are etched into their blade. Think Warcraft III when you think about the possibilities of abilities for the Death Knight. I say possibilities because things are still in process for the development of the class and there will be (of course) a lot of testing for balance done. Given that this is our first Hero class we're unveiling, you can be sure we're going for the coolest of the cool for this class.
Runes and Mana pool
Is the Death Knight going to have a mana pool? The pictures from BlizzCon show spells using mana.
The runes they inscribe on their weapons are their mana pool (of a sort). As they use their abilities, the runes will be exhausted. After a period of time the runes will refresh. So, they don't have a traditional type of mana pool. You'll most likely see some screenshots of it, but the Death Knight (at current) has three different types of runes that they can inscribe on their weapon: Blood, Frost, and Unholy. Runes can only be inscribed out of combat ( all of this is subject to change as development continues.)
The rune types don't dictate a new damage type, they're simply a resource that spells and abilities will use when you activate them. The spells and abilities won't necessarily be "blood, unholy, or frost" damage.
Imagine it being three different energy pools which you can change the size of before you get into combat.
There's no final decision or announcement on how multiple weapons would be etched, or how weapon switching may affect the etched runes. What has been discussed to some degree is that the main-hand weapon would be etched and that's it, but it's too early to say what would be allowed in the final game.
Also, we haven't decided on what level you will start as your Death Knight, but it seems that it should be easy enough for someone who has levelled a character to level 80 and done the quest to be able to adapt to a Death Knight fairly well. It's not quite the same as handing it over to someone who is still learning the game. There are lots of opportunities for someone to solo and group to learn how to play the class.
It's true that the current plan has been heavily discussed to go ahead with all races, but no final decision has been made. After talking to Chris he wasn't totally sold on the idea. Both are technically correct in that the current plan of some of the developers is to go ahead with all races, there's been no final decision made and no real work has progressed to force a decision just yet. The FAQ is more accurate at this time, in my opinion.
Lore-wise there's really no reason why a specific race couldn't become a Death Knight.
All races can be challenged mentally and physically, their will dulled, and be cast down into madness ... and darkness. Their souls drawn into the runeblade they wield.
Compared to other classes
They are not planned to be more powerful, them being a hero class simply means they're introduced at a higher level, and in general will reference the more powerful hero types of Warcraft history.
It's our intention to integrate them into the current classes, balance them appropriately, and have them provide an additional - but equal - role in groups and raids as compared to other classes.
Will there be other hero classes?
Yeah, well that kind figures in to the new definition of what a hero class is. It allows us to add new classes that start at higher levels, keeping players relatively kept up as we continue to release expansions and raise the level cap, and ultimately it really fits to use the heroes from past Warcraft games as inspiration. Some other hero classes could definitely be things like Demon Hunter, Blademaster, etc.
If you want to get in on the discussion or read more, head over to the Death Knight FAQ forum post.