Attack Power: Attack Power will be completely removed from gear, to be replaced by agility and stamina. For the most part, this should have minimal impact on us, as the best death knight DPS gear already uses strength. That said, some death knights do end up using attack power gear just because they haven't seen a better drop, especially if you're a new 80 running, say, the coliseum or the Icecrown 5-mans. What this means is that if you have any attack power items laying around, you'd best get them replaced by strength gear before Patch 4.0 gets here.
Armor Penetration: Armor penetration will be gone from gear, to be replaced by haste and critical strike rating. Armor penetration is currently a pretty powerful stat for both blood and unholy death knights, and not completely worthless for frost death knights either, so chances are you have a decent amount of it on your gear. It'll be gone, but chances are we'll see it on mastery again anyway.
Haste: Haste will be reducing the cool down of our runes in cataclysm. The exact effect this will have on DPS should be interesting. Death knight DPS involves not only getting out attacks quickly, but efficiently as well. We have to make sure our weapons strikes fall on a target loaded up with diseases in order to optimize our DPS. Therefore, we'll need to determine if the haste allows our runes to cool down fast enough that we can get in extra strikes before our diseases need to be refreshed. It's going to take some math.
Mastery: Mastery is a new statistic that will show up on gear and will give you extra passive stat bonuses based on your class and spec. Of course, since death knights can tank or DPS in the same trees, we'll have to see how Mastery determines whether a death knight is tank or DPS. As far as what stats to expect here, it may differ a bit by tree, but I expect we'll see armor penetration, critical strike chance, and haste off our mastery. Whatever we get, the devs say they'll have more information on mastery for us soon.
Plate DPS Gear: It's worth noting that stamina and armor class are going to be bumped up for classes across the board. We should still be relatively non-squishy, but there will be less of a gap between us and, say, a mage. It's also worth noting that wearing your proper armor class will provide a mastery bonus. This will probably just be icing on the cake for us, since only plate DPS gear will have the strength we need in the first place, but it's worth a mention.
Ratings: Combat ratings, such as hit and expertise, will not "cap out" as easily in Cataclysm. You'll see higher raiding tiers of mobs being harder to hit, for example. This should allow the developers more freedom to itemize, and should mean that all that hit rating off every other drop might actually be useful. On the more annoying side, this could mean we'll be missing our target a bit more with swings and spells, which means more dropped rotations and less DPS. Again, though, we'll need more solid numbers to be absolutely sure.
Weapon Skill: I'm sure most of us old time death knights have our weapon skills all properly trained to at least the 390 skill range now, but for new death knights, this is going to be nice news: weapon skill is gone in Cataclysm. You'll be able to wield the weapons your class can wield without having to kill grey mobs for 2 hours. For everyone who had to skill up maces from 0 to wield their nice new Titansteel Destroyer, this one's for you.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.