Be you in any class, it is important for you to gear for your character. In particular, it is critical component for a Death Knight. It seems to say that Death Knight is a class whom is likely to make use of use the old Dirty Harry saying "Do you feel lucky, punk?" Death Knights are not there to make friends, what they do there are to kill and intimidate, preferably in that order. The bigger and more impressive the gear is, the better you make for yourselves.
   In the World of Warcraft, you spent most of your time in looking for, collecting, making, buying, questing, or raiding for gear. Just when you get a good piece of gear, you will either level past its usefulness or find a better piece of gear to aim for. Gear and gear progression is what ends up keeping most players long past reaching the level cap in the game.
   Due to the fact that Death Knights start at level 55 upon character creation which are different from other classes for gearing up. What's more, they start the game with the green quality gear. They are able to earn a better set of rare gear through completing their starting zone quests. This second set of gear lasts well into a Death Knight's leveling. Therefore, as a Death Knight, there is very little that you will end up replacing for the first 5-10 levels.
   It is important for Death Knights to choose the gear and stats for them to rely on what talent spec you are using. The Death Knights do very well.
They can kick out enough DPS to make almost any other class quake in their boots. The stats are vastly different than what you need to tank, which you require to do damage. This means a whole different set of gear. On your damage gear, the most important stats are Hit, Strength, Attack Power, Critical Hit, and Expertise.
As a Death Knight, you should be looking primarily at plate gear and be open to mail armor if it has significant stat upgrades from your current plate gear. But as a DPS Death Knight, the goal is to cause enough damage to kill your opponent first. While it's nice to be able to take a few hits, if you get a big enough damage output increase it's worth sacrificing some survivability for it.
For tank, the goal of the Death Knight is to piss everything off which they focus on you. You need to cause enough damage and be able to absorb or avoid enough of the incoming attacks while staying alive if you do this. On gear, Stamina, Strength, Defense, Expertise, Dodge, Parry, and Armor value are the most important stats while tanking.
In player versus player, burst damage output and survivability are absolutely necessary. For most of the Death Knight, they focus on burst damage when entering into the PvP combat. That is to say, your gear will focus on the Strength and Critical Hit. It is a tricky affair to find the right balance in PvP. As you gear up for survivability, your burst damage capability goes down; as you gear for more damage output, your survivability goes down. Finding the balance will take time and depend on your situation and play style.
Death Knight can choose many different weapons range from one and two handed swords, maces, axes and so on. Even tanking, they are looking for the biggest and meanest beat stick around. To some extent, the Death Knight dual-wield handed weapons but it is avoid to use because it is their forte.
The damage and speed are the most critical stats with weapons. It is obvious that the other stats bonuses come into play on a weapon, but they are secondary considerations. The priority on weapons goes first to the base damage they cause. This means that slow weapons are better since they cause more damage per hit. Secondly, you want to look for a weapon with Strength or Attack Power, and only after that, Hit or Critical Hit rating.
Death Knights tank using two-handed weapons and all the same choices apply to choosing a weapon with which to tank.