The relatioship between warriors and death knights is an interesting oe however. They are very much twins, their abilities being mirror images of each other, though distorted by the eddies in the lake. Just consider it for a moment, warriors have many abilities that I personally envy; charge, Disarm, and shield. In fact, they are the class of choice against heavy melee fights. For instance, disarming the orc commander, first boss of heroic Nexus makes that whole fight so much more easier, mitigating a whole bunch of damage.
Death knights on the other hand are a class, designed specifically to be the bane of spellcasters. Though, to my grief, failed in that somewhat. We use two-handers, instead of a shield, use Death Grip instead of Charge, and have powers like Strangulate that prevents a caster from using spells. Thanks to their shield, warriors mitigate more damage than death knights, but to our advantage, we put out a lot more of it.
So, death knights are against spellcasters what warriors are against melee opponents. Many have speculated that Blizzard will publish a second hero class, specifically against melee fighters, but this is why I do not believe they will, or at the very least, they definitily should not. We already have a class for that and it works beautifully. The only big complaint I have about the synergy of these classes is the limitatio of stragulate.
A warrior can disarm a various collection of both mobs and bosses. However, death knights do not enjoy the same power over casting bosses. So far, I have yet to find a single one that I could affect by using strangulate, or even Mind Freeze. They are both more PVP oriented abilities, or at least only useful against mobs. But perhaps, once Blizzard has managed to please the public's demand to nerf death knights until we're wearing hong kong made armor pieces and picks for weapons, they desire to actually do something for us, for a change.
All and all, both are solid classes and personally, when death knights first came out, during which information and details about the nuances of playing the class was scarce, looking through the tons of articles written on warriors really helped me personally. Gear is almost identical after all, aside from the shield and block stats, and the tanking techniques do not really change much from class to class, only the way you execute them.