We know that Death Knight spells and abilities operate through a new resource system unlike Mana, Rage, or Energy called Runes. They also utilize a secondary resource system called Runic Power, which is kind of like the bastard child of Rage and Combo Points. Runic Power diminishes over time if not used or generated (like Rage) and can be spent to unleash special abilities, with more Runic Power usually yielding more devastating effects (like Combo Points). Runes aren't as straightforward as current resources, such as Mana -- a spell costs X Mana and that's about as complicated as it gets.
There are three different Runes (four if you count the Death Rune) and six Rune slots, and players get to choose which kind of Runes to activate, or etch on their Runeblades. It's a more complex system, and players will adaptively have to change up their Runes or settle on a combination that works best for their spell cycle. The creation of the Rune system is very daring of Blizzard and shows that they're not content with keeping the status quo. The Rune system in its current form seems so interesting that I'm inclined to create a Death Knight simply to play the Rune management mini-game.
Here come the corpse corps
Blizzard doesn't stop there, either. Death Knights use another resource that shows off their developers' seemingly newfound mastery of environment code -- corpses. In the game currently, corpses are mere visual indicators for dead players and NPCs; they have no use other than to clutter the environment and take clever screenshots. The Unholy abilities Raise Dead and Corpse Explosion both use corpses to gruesome effects.
Corpses as spell reagents? That's brave as hell. Corpses are not a controllable resource... it requires awareness of and interaction with the environment. Players can't necessarily control where mobs or other players die, or even how many corpses there will be. That means casting Corpse Explosion takes a bit of planning, maybe even baiting opponents into the vicinity of a corpse. It's a different playstyle. Imagine how much fun Alterac Valley would be, with corpses all over the place. With Corpse Explosion's 20 yard range, players should probably think twice about walking near corpses in the expansion. EDIT: Notmut correctly reminds me that Cannibalize also uses corpses. Good call. Still, an offensive spell utilizing corpses is an interesting concept.
Diseases please us
Death Knights also use a debuff Dispel Type previously only the domain of NPCs -- disease. This means that Blizzard didn't just create something new, they also looked into using mechanics that have been in the game since the beginning. The curious thing is, Death Knights don't simply have a disease-inflicting spell... no, that would be too simple for the Wrath development team! Instead, disease is a side effect of many Death Knight abilities, the primary one being Plague Strike, a melee strike that inflicts disease.
The Unholy tree has talents that inflict additional diseases, such as Blood-Caked Blade or Unholy Blight. The Ghouls that Death Knights create inflict disease, as well. These will likely be unique debuffs in order to maximize those Death Knight abilities that take advantage of the number of diseases on a target.
Better focus, better talents
If there's one thing that you will immediately notice with a quick perusal of the Death Knight talent tree, it's how focused and synergistic they are. Blizzard only began to hone this focus in The Burning Crusade, where they designed talents that catered to different playstyles. A Subtlety Rogue, for example, will play completely differently from an Assassination, or Mutilate Rogue. The mobility is different, the focus is different, and even the attacks are different. With the benefit of experience, Blizzard seems to have designed the Death Knight with such clear focus that it looks as though differently specced Death Knights will have completely unique feels. This is the case with many classes currently, but the new design seems even more focused now.
The Blood tree, for example, has several talents that take advantage of killing an opponent that yields Honor or experience. This means Blood-specced Death Knights actually need to time their Killing Blows better. The Unholy tree focuses on disease, and have several talents that work synergistically with the stacking of diseases. Unholy Death Knights will be concentrating on applying as many diseases as possible, sort of like an Affliction Warlock. The Frost tree, meanwhile, is all about slowing or freezing opponents and speeding up the Death Knight's own attacks.
What makes the new design so focused, furthermore, is the sheer number of single-point talents in each tree. These new abilities, passive and active, ensure that the trees have a completely different feel from one another. Many talents in the existing classes are mere improvements over baseline abilities which, although sometimes necessary, is somewhat boring. Death Knights have those, too, but the number of single-point talents in the trees allow for great flexibility in choosing talents. Most classes now have 4-5 single-point talents per tree. In comparison, the Death Knight's Unholy tree has a whopping 7 single-point talents in seven tiers, 8 if you count the 51-point talent Unholy Blight. The Blood tree has 5 while Frost has 6, not counting the 51-point talents.
I note single-point talents because these are quite often good investments of a talent point, particularly active abilities. These are new spells for the cost of one talent point. I like that design philosophy -- Blizzard isn't afraid to give different specs a plethora of their own unique spells. I wish that they had been this creative from the start. The synergies in the current talent trees of existing classes just don't have the same... excitement as the Death Knight's. Blizzard went all out with their Death Knight design, and although I'm pretty sure a lot of this will change before Wrath goes live, it looks really good so far.
What this means for other classes
The good news about Blizzard's brave new design directive is that they can apply it to existing classes. Looking through Wowhead's talent calculators, it seems like they're certainly being creative about the 51-point talents. On the other hand, it would be great to see the developers work on the lower tiers of the talent trees, and not just 41-points and above. More single-point talents across the board would be an awesome, if unlikely move.
Overall, learning more about the Death Knight class only makes me more excited for Wrath of the Lich King. It really seems like Blizzard is keen on innovating and not simply rehashing old talents and abilities. They're making really good moves with the existing classes -- such as combining redundant totems like Strength of Earth and Grace of Air and scrapping below par spells like Rockbiter Weapon. If they streamline all the classes this way, it could pave the way for newer, more exciting abilities. The Death Knight looks like the product of a more experienced development team. Right now, at least, it looks like Blizzard is very much on top of its game.