If you missed out on the Beta test, or you didn't get to play that much, I have some advice that might help you understand the way a Death Knight tank is going to interact with your group.

Trash:

It takes a few GCDs to establish threat on a trash pack. The Death Knight is likely going to open up with an Icy Touch and a Plague Strike on the "main mob". This will apply Frost Fever and Blood Plague (and Ebon Plaguebringer if he's Unholy). The Death Knight will then Pestilence to spread the diseases from the "main mob" to the rest of the mobs. The remaining three runes will likely be used on Death and Decay. Death and Decay can be used before the Pestilence combo for a little better initial threat, but if you spread diseases first it will hit harder.

What does this mean for a healer?

Give it a couple of GCDs before you drop a heal. Don't HoT the Death Knight up before the pull, or you're going to get agro on everything but the "main mob". When you see the Death and Decay go out you can likely begin healing. The Death Knight will probably use a short cool down defensive ability to mitigate damage at the onset of the pull anyways.

What does this mean for a DPSer?

If you're doing single target DPS you need to wait until the "main mob" has at least both diseases on it. If you're doing AOE DPS you should wait until everything has both diseases on it. Remember that Death Knights do not have an AOE Taunt so if you pull threat on multiple mobs they have no reliable way of grabbing everything off of you right away.

Bosses:

It takes about one rotation to establish threat on a raid boss. In that time the Death Knight will use Icy Touch and Plague Strike again, and follow that up with two Blood Strikes, and an FU ability (Obliterate, Death Strike, or Scourge Strike). This rotation will likely give the Death Knight enough Runic Power to use a Death Coil, Frost Strike, or Rune Strike as well. Healers should be able to heal right away, and DPS should probably wait until near the end of the first rotation to begin DPSing. After the first rotation threat should no longer be an issue. Misdirection and Tricks of the Trade add even more threat in a raid setting.

Cooldowns:

Death Knights have a lot of defensive cool downs. Unbreakable Armor, Lichborne, Bone Armor, and Icebound Fortitude to name a few. These are on short timers, and one of them will be up for every trash pack in an instance. They can be stacked which means during a strong Frenzy or Enrage the Death Knight will not take as much damage as you might expect. In combination with trinkets that provide mitigation cool down effects a Death Knight could theoretically "roll" all of his cool downs in to the next one so as to always have one up. Likewise the Death Knight can stack these for increased survivability with reduced uptime.

Hopefully this very brief overview helps you in future groups with Death Knight tanks.