It's one of the classic truths of the Wow universe. DEAD DPS DO NO DAMAGE PER SECOND!!!
Yet my much-revered site Elitist Jerks uniformly and reflexively sneers at any suggestion of talents or even playstyle that smacks of self-preservation. All discussion in the dps forums is about the best spec, the best rotation/priority, the best gemming and gearing, to put out the most single target (with the occasional nod to aoe) dps possible.
That's all to the good, however, it remains that most guild-first bosskills are accomplished with a handful of raiders alive at the end of the fight. Being able to do 10,000 (as opposed to 9k) dps on Saurfang is merely hubris unless it also serves to defeat a heretofore unbeaten baddie.
I pride myself on being one of the last Draeni standing at the end of fights like Putricide, where there's a tank, a healer, and.... me. On our first Valithria 'win,' our pally healer bubbled the last few seconds of the fight and topped off the lovely dragon, but I had gone down mere moments before, after the last tank and raid healer had already expired. DKs have a unique set of tools, and if we're dying early, we're failing to optimize our performance as a class.
I spend the vast majority of my time discussing the Holy Grail of max dps, but today let's talk about surviving.
Threat Management. Death Knights do lots of dps. DPS is one way to get threat. DPS DKs that have more threat than the tank go splat! See how that works? That being said, any DK that pulls threat off the boss is a fool and repeat offenders need at least a talking to and if done often enough, a demote to non-raider. First of all, the majority of DPS DK specs incorporate Subversion, a talent that at 3/3 reduces threat by 25%. With this talent, and the lovely tools tanks have, it should be simple to avoid pulling threat. IF you pull threat often, the obvious approaches should be utilized. Wait a microsecond or two before unloading on the boss. Duh! Of course, be aware when encounter dynamics have a drop in agro so that you don't start dps too soon. As usual, if there are adds in a fight and you get agro from one of them, don't panic - run it to the add tank. Casters may need to be silenced first before they'll follow you - Mind Freeze and Strangulate work wonders here, and perhaps judicious use of Death Grip.
TOC and Icecrown are full of fires. As the euphemism goes, "Do Not Stand In Fire." From the Northrend Beasts to (I'm assuming at this point) the Frozen Throne, there's avoidable damage that will kill you if you don't pay attention.
Example: Professor Putricide. Malleable Goo launches into the air from time to time. If you're in melee range of the boss, it shouldn't target you, but if you're chasing down an ooze to kill it, you may be targetted; don't run into it! If you must, know that it's incoming and use a cooldown to minimize the damage. Also, the green slime on the ground must be avoided. Don't stand on Choking Gas Bombs, which not only apply a dps-crushing hit debuff, but also explode after 20 sec. If your health is low, don't run to a Volatile Ooze that's about to hit someone. And so on. It's a survival and dps fight, and you need to be alive to beat the enrage.
So, be aware of the different things in the fight that are potentially harmful, and be careful! Don't do anything stupid to get yourself killed! Run around that slime, stay a safe distance from the ooze, etc.
Most boss fights also have those events that damage the whole raid and avoiding them isn't possible. Koralon does his fire pulse thingie, Icehowl knocks the raid back, Blood Queen Lana'Thel has a nasty aura (Shroud of Sorrow), and so on. Now, these shouldn't worry you very much. They're designed to challenge the healers and still allow the raid to live. If the clothies can survive it, you can too - on most occasions without any help. However, be aware of when these events occur. Often, it's worth using a cooldown to prevent your health from dropping too much and to keep the worst of the strain off the healers, and to keep some small amount of damage from finishing you off immediately after. When Festergut uses his Pungent Blight ability, I always use Anti-Magic Shell to take the worst of it. Know the type of raid-wide damage, and be prepared to deal with it.
Surviving; Putting it all together.
Let's look at a wonderful and pleasing case of Death Knight survivability: Heroic Twin Val'kyr. Like most guilds on heroic, my group uses the 'door strategy.' For those not familiar with it, the raid clumps up in the doorway you enter the arena through. EVERYBODY selects the dark color buff. 4-5 players are assigned to go light and 'soak' the light orbs by running around and grabbing them all when they're out. Then you simply dps the Twins down, without switching colors.
Of import here is that by not switching colors, the ENTIRE raid gets hit with the opposite colored Twin's Vortex. This does about 60k damage. That's 60,000+ damage! And guess what, you can easily survive this event without a single heal. Other classes get the shields, or are targeted for the emergency heals and swiftmends. DKs need nothing.
Let's walk through it:
Assume the decently geared and buffed dps DK has over 30k health.
Vortex should kill them twice over, even starting at full health.
Anti-Magic Shell absorbs 50% of your health in damage. 45k damage remains.
Icebound Fortitude reduces the incoming damage to 36k.
Death Strike (much less the blood tree improved version) heals for about 4k or so; you should be able to use it twice, restoring 8k health.
Healthstone heals for about 5k.
Health potion (if you use one) heals for about 3.5k.
Without a single heal, in decent but not great gear, you should survive that scary Vortex with about 10k health!
Myself, I have around 34k health raid buffed, and I use my Draeni racial, which heals about 18k over 10 seconds with a decent amount of attack power. I usually get at least a few HOT ticks, a Circle of Healing, and a Chain Heal, so typically after Vortex is over, I'm only a few thousand health below full, if not completely topped off. :)
Granted, you're burning all of your cooldowns at once, but my oh my it's good to feel untouchable!
Surviving big bad raid-wide events, avoiding silly killer mistakes, not pulling agro, and wonderful defensive/self-healing cooldowns add up to a longevity that allows you to keep on hittin till the boss is sittin!
EDIT: as a side note: Icebound Fortitude change may or may not stay in the game, but it's now providing 10% more damage reduction (about 4.5k less damage in our example), and Bone Shield for Unholy DKs reduces that damage another 20% (another ~9k reduction). Wewt for survivability!