As we said above, they will almost definitely not stay the same on live, and may not even be the same by the time we get access to beta. That said, the way they're designed offers some interesting insights into how Blizzard is designing class balance.
Necrotic Plague, which replaces Blood Plague with a disease that stacks with itself and automatically spreads to others, has a lot of great potential for a frost death knight. Since we use Howling Blast as a normal part of our rotation, Necrotic Plague would pretty much remove the need to spread diseases on a normal fight. Thus, we not only get some great flavor, we get something that shores up a small weakness in a certain spec. Other specs can take it for increased damage, as well. Sindragosa's Breath is nice callback to the original Unholy Blight, converting runic power to AoE damage. If it stacks with Scent of Blood, it may end up being a great trash threat tool for blood death knights, especially. Defile works similar to the Lich King ability, in that it gets bigger the more damage it does, and promises if nothing else, to just feel amazing to use. It replaces Death and Decay as well, so it won't clutter your bar with an extra button.
In tandem with the above, Blizzard announced that people wouldn't be getting new spells in the level 90-100, but instead, existing spells might be tweaked, gaining new effects or even going passive. If you've been reading this blog, you probably know where I'm heading with this: Passive Army of the Dead and passive Soul Reaper. You could immediately firm up the death knight rotation and shed some buttons by making Army of the Dead a Wild Imps style passive, also giving them some fun flavor by providing a semi-permanent army. Soul Reaper could return to its Merciless Combat roots, possibly making its current debuff become a random proc when attacking a create below 35% health. At the same time, death knights aren't hurting for hotkeys as much as many other classes, so it's possible we'll simply see skills adjusted. For example, if Soul Reaper stays its own button, perhaps it will be extended to work below 40% health.
Gaming the System
Class changes, on the whole, are still pretty nebulous and likely to stay that way for some months. That said, a lot of basic systems changes appear mostly set in stone, and they definitely promise interesting consequences for death knights.
For Hybrid classes, the primary stats on armor will be changing based on spec. This won't actually affect us much, because both death knight tanks and DPS use strength. What is more likely to affect us is the removal of hit, expertise, dodge, and parry as stats on gear.
Dual wielders should especially rejoice at the loss of hit rating. In the past, it's been impossible to "cap" hit for dual wielding. With over 30% hit rating needed to prevent all dual wield misses, it isn't worth it. You usually just settle for the special attack cap and deal with it. Now, that's gone away. Even two-hand users should love this, though. You won't be frantically reforging away 12% total hit on your gear and finding it's nearly impossible. This is a change a lot of people have wanted for some time, and one that's very welcome.
This does bring up some interesting questions for tanks. At this point in time, Mastery is the only remaining traditional tank stat that will appear on gear. This suggests that, especially with the removal of reforging, critical strike and haste will have to be made useful for tanks in some way. Thus, we could certainly see some cases where a DPS death knight whose spec favors mastery switch to their tank spec and running a few quick dungeons without changing gear, because tanking uses the same stats. That said, the serious dual-speccers will still want a separate tank and DPS set so they can use unique enchants and use the best stats for that specific spec. It's nice to have the option for a little casual tanking or DPS, though.
Another thing to watch out for will be the new secondary enchants, such as run speed and cleave (which lets your attacks hit multiple enemies). It's going to be hard to speculate how those will work out. Unholy may be able to eschew run speed because of Unholy Presence, but frost and blood is likely going to want at least some. You should never underestimate how moving quickly to get to the next mob (or to get out of the fire) can benefit you, especially as melee. Cleave's issue is going to be that it's going to be useless on single target boss fights. Of course, as a secondary stat which will have a separate item budget from primary stats, this may end up being a non-issue, but I can see a problem with people avoiding cleave gear unless it gets some sort of single-target boost. Then again, that's probably a more universal issue than a death knight specific issue.
The level 90 boost is interesting that it makes death knights slightly less unique. Whereas death knights always skip 55 levels, now any class can skip the first 90 levels. Whether you agree with the feature or not (I do, for the record), it does reduce death knight uniqueness just a bit. Still, it's not a huge deal in my mind. I am actually considering using my boost for another death knight, just to have another one for experimentation and fooling around. We'll see how it goes. A lot is likely going to change between now and when the game goes live, and we'll have a lot more changes to cover soon enough.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.