Strength and Attack Power: Strength is the number one stat for most types of Death Knight DPS, and is hardly a slouch for tanking either. 1 strength converts to 2 attack power, and that attack power scales into spell power for the purpose of a Death Knight's spells, which include diseases and non-weapon attacks such as Death Coil. In addition, 4 strength converts into 1 parry rating.
Long story short, Strength is going to be your mainstay for DPS, and it's no slouch for tanking either, providing both extra parry rating and extra threat. You'll probably end up taking attack power gear every once in a while, especially rings and amulets, and it will work fine for you, but remember that strength does go that extra mile as well.
Critical Strike Rating: This is easily the second most important DPS stat for a Death Knight, especially since it affects both melee and spell critical strike percentage. Death Knights need critical strikes to activate talents such as Bloody Vengeance, Killing Machine, and Wandering Plague, and everyone appreciates a good Death Strike crit when they're low on health. Don't neglect this statistic, especially if you have one of the above talents or a talent that increases your critical strike damage bonus.
Hit Rating: I cannot emphasize the importance of hit rating enough. You can have all the strength in the world, and it doesn't matter much if you can't hit the broad side of a dragon. One of the best ways for a tank to up their threat level is to get some hit rating. At the same time, once you have enough hit rating, getting more won't do you much good.
The problem is, right now, there's some disagreement over how much hit one needs. Traditionally, one needs 5% hit rating to hit like-level mobs or PCs with 2-handed weapons and special attacks, 9% to hit boss-level (level 83) mobs with the same, and 17% to hit boss level mobs with spells. However, some are claiming that the new boss-level hit cap is now 8% for melee special attacks, and parses of damage meters seem to be offering conflicting information on whether this is true or not.
For now, I'd recommend that you try to get around 5% melee hit rating while leveling, grinding, or doing 5-mans, and somewhere between 8-9% melee hit rating for raiding until we know more for sure. At 80, 32.7 hit rating equal 1% of melee hit, and 26 hit rating equals 1% of spell hit. That means you'll want somewhere between 270-296 hit rating for that raid cap.
If you want to try to cap out your spell hit rating, you should probably take Virulence in the Unholy tree, then aim for the approximately 365 spell hit rating you'll need to make up the other 14%. Dual wielders have even further to go, with approximately 28% before they cap out white damage hit percentage. If you focus on getting to the spell hit cap and take Nerves of Cold Steel, though, you might be able to squeak through with less.
Remember, in all of these cases, that if you're grouped with a Draenei or are a Draenei you can lower your needed hit by 1%, while a Shadow Priest's Misery or a Moonkin's Improved Faerie Fire can lower your needed spell hit by 3%.
Overall, Hit rating is something that theorycrafters are going to want to watch much more closely. There's evidence that some calculations have changed pretty drastically, but debate continues as to just how much things have changed. In the meantime, it's a good idea to keep hit capped anyway. It's better to hit than to miss.
Defense Rating: If you're planning to tank 5-mans or raids, especially at the Heroic level, you'll need to get uncrittable via defense, which at level 80, will mean you need 540 defense skill. This means that at level 80, you will need approximately 690 defense rating to gain critical strike immunity against level 83 (Boss level) mobs.
The unfortunate problem with this is that we're definitely a little bit harder to gear for defense that other tanks, having neither the benefit of a talent like Survival of the Fittest nor the ability to equip a shield nor defense itemization for 2-handers. Luckily, this is something that the devs have acknowledged, and have said they may fix in the form of new defense Sigils and runeforgeing enchants into the future.
In the meantime, some people are taking creative measures to get their defense to the magic number. For example, since defense trinkets are in short supply in Wrath until the harder dungeons and raids, many are heading back to Burning Crusade to get Dabiri's Enigma from the Netherstorm quest Dimensius the All Devouring, while others are even headed into the Shadow Labyrinth to solo Blackheart the Inciter in hopes that he'll drop the Adamantine Figurine.
Parry Rating: The more Parry rating you have, the less you get hit. As a tank, this is good. Look for armor with this stuff on it once you have your defense rating and a good health pool. If you've picked up the Spell Deflection talent in the Blood tree, you have even more reason to grab parry rating.
Dodge Rating: The less you get hit, the less chances you have to die. That's really the best way to describe why dodge rating is a good idea for a Death Knight tank. This is especially true of Unholy Death Knight tanks. Their major tanking cooldown, Bone Shield, provides a major amount of damage reduction as long as a bone is up. A successful hit removes a bone, and each bone removal is followed by a short period before another hit can remove a bone -- most Theorycrafters estimate that period at between 2 to 4 seconds. Thus, the more you dodge, the longer the shield stays up.
Stamina: As a tank, you need stamina. The more health you have, the longer you survive. It's a simple idea. A larger HP pool works against both physical and magical damage, which is also a plus. Even as DPS, you shouldn't neglect Stamina completely. AE and Environmental damage or loose adds could spell your doom if your health pool is too low.
Armor: If you're a tank, you definitely want Armor. For Death Knights, it's even more important to find decent high armor pieces since we don't have a shield. The more armor you have, the more you'll get when you switch over to Frost Presence, as well.