If you think of a death knight tank as a plane, I find it helps to picture this tank as a gnome who are often punted, perhaps in that aerodynamic tier 9 gear with little wings. Where was I... oh yes, flying tank.
Imagine the tank's health to be their speed and their threat to be their altitude. Now anyone with a basic working knowledge of planes know that if you pull up to gain altitude while at too slow a speed the plane will stall and being to free fall toward the ground. The only way to avoid a crash is to level out and build speed. In the death knight's case, stop building threat to regain health.
The analogy takes a comic turn when you picture a healer as a rocket strapped to the flying gnome, providing an extra boost of speed when needed. An afterburner of sorts. Yeah, healers just got cooler.
Death knights are unique in comparison to other tanks due to the fact they use the same resources to mitigate and heal through damage as they do to cause damage and threat. This is most true of a tank specced into the blood tree. There is a ton of survivability there, but it comes at a higher price in runes than other talent trees.
So here we have our little squadron of plaople (plane people?) with the tank in the lead and the healer strapped on for the ride. The tank loses speed and the DPS fly right on above their tank above the poorly engineered downward facing radar jamming transmitter on the tank. They enemy spots them and proceeds to shoot them down. Up until that point the enemy was only aware of the tank.
So something DPS should be more aware of when working with a death knight tank are more abrupt fluctuations in threat generation and in turn should give the tank a slightly larger lead on threat and/or keep a closer eye on it. This is especially true of AOE encounters where the first thing to get axed are the more expensive AOE threat generation abilities.
The healer also needs to keep in mind as they have just somehow magically become attached to all the planes whenever they chose is to not boost any DPS past the tank. In other words in order to keep your tank making threat don't let their health play second fiddle to that of a DPS. This is really only an issue when healing someone who has pulled off the tank.
When someone pulls off a death knight tank and that death knight is forced to use runes to protect themselves they are less capable of regaining aggro aside from a taunt. Luckily we have one and a half of those, I count death grip as a half since it doesn't match threat like a traditional taunt. The death knight still need to expend resources in addition to a death grip to retake aggro permanently. So if the DPS doesn't stop damaging while that process is taking place they will be stuck with that mob and compound the problem as they require more heals and the tank must concentrate more on themselves.
Compare that to a similar situation on my bear tank where all I sacrifice are a couple global cooldowns and it is a night and day difference. Both have their own strengths and weaknesses.
* No gnomes were punted in the writing of this post.