1) Hold off on that AOE for just a second if you would. Death and Decay is less like a Thunder Clap and more like a Consecration: a pulsing mass of energy on the ground that steadily builds threat. It is not a huge spike of threat, and it will not build threat if an enemy dashes into it and you immediately shoot them, causing them to run back out. Sure, its puny-looking damage is amplified by our talents and AP and whatnot (plus diseases, for Unholy DKs with Ebon Plaguebringer), and it has an enormous threat component when combined with Frost Presence (something ludicrous like 250 Threat per point of damage), but here's the part a lot of AOE happy boomkeks and volley launchers don't seem to get: it isn't going to build any threat if you don't for Heaven's sake let me hit it first. Let me put down my D&D and spread my diseases - my other quick method of building constantly ticking threat, and you'll know when it's down because you'll see black lines briefly connect through all the trash - and look at Omen, and then go ahead and unleash your great thundercloud of lightning-y doom.
One thing a lot of people are not aware of is that most of the DK specs are relying entirely on D&D for AOE threat. Unholy tanks used to be able to count on Unholy Blight as well, but our giant cloud of buzzy fleas is now simply a DOT applied when we cast Death Coil. Frost tanks can use Howling Blast, but that's got its own cooldown on top of our normal rune cooldown; they've probably got it easiest. Blood tanks are a lot like warriors using Cleave, only they can't add an extra target via glyphs - Blood AOE consists of tab, Heart Strike, tab, Heart Strike, tab, Heart Strike. It's kind of hectic.
In short: if I see one more hunter target an enemy on the other side of D&D and expect a simple run through the red glowy circle to out-threat their shots, or a druid who pops a thundercloud before Death and Decay has ticked once, and then bitches at me because they're getting their face beaten in, I am going to scream. No lie.
2) If we are silenced, please dispel it. This is something a lot of DKs are not aware of, as well as a lot of people around them - hell, I didn't figure it out until I started tanking. If we are fighting a mob who is capable of silencing us, and you notice we're silenced, please do something to get rid of it if we can't do it ourselves. When silenced, we lose access to:
possibly Mind Freeze
possibly Rune Tap
possible Vampiric Blood
The list goes on and on. Essentially any spell-like ability is affected by silence. In short, if a DK is silenced, our list of threat and survivability tools is reduced to Plague Strike, Scourge Strike (unholy only), Blood Strike, Frost Strike (frost only), Obliterate, Death Strike, and Heart Strike (blood only). We lose the ability to lay down an AOE (any of them at all), we lose the ability to taunt, and we lose our biggest source of magical protection and free runic power.
So please, if you see your DK tank get silenced, and it's a kind of silence you can dispel, please do!
(P.S. Casters with silence and hunters with silencing shot, if you really want to shut a death knight down in PvP, I guess now you know what to do.)