I hear a lot of complaints about how awful most Death Knights are, and I share your pain. My main is a DK. But unlike a lot of other DKs, I spent a lot of time learning how to play the class correctly, read a lot of web sites, and picked the brains of a few of the good DKs I know.
One of the biggest causes of this problem is the fact that DKs are extremely over-powered at the lower-levels (55-78). This means that there is little reason for people to learn how to get the most out of their DK. When you are questing, you can just mash a few keys and down any mob in a matter of seconds. If you haven't chosen talents like a complete moron, you can kill 3 or more mobs at once and be at or near full health after the encounter.
However, when it comes to more advanced play (instances and raids), a Death Knight is actually one of the more complicated classes to play correctly. It's not like some classes where you can just spam two or three keys repeatedly. I have no fewer than ten abilities that I use in my regular rotation, and it requires a lot of planning and attention to detail to know exactly what to cast and when. You have to know which of your spells require runic power, blood runes, frost runes, or unholy runes, and you have to know when each will be available.
A typical AoE attack with my unholy Death Knight goes something like this:
- Death and Decay: Open a can of whoop-ass on the entire group. It must be noted though that you can only use this ability if you have a good tank and you give him a couple of seconds head-start to grab and hold aggro. This is a high-threat ability and will immediately draw aggro if you have a mediocre (or worse) tank. If you are unsure, better to forego it.
- Plague Strike, Icy Touch: Get your diseases up and running on the main target. Each one is a 20 second DoT.
- Pestilence: Spread the diseases to all other targets in the group.
- Unholy Blight: burn some runic power with another AoE attack.
- Scourge Strike, Heart Strike: Kick some butt on your primary target, using up any remaining runes and building more runic power.
- Death Coil: cast repeatedly until runic power drops below the necessary 40.
That's eight different abilities, and that's just the initial part of the attack. Now comes the ongoing attack:
- From that point I go into a rotation of Scourge Strike (1 Unholy, 1 Frost), Heart Strike (1 Blood), and Death Coil (40 runic power) as each one comes available.
- I keep an eye on the disease durations. When they are getting low, it is time to cast Pestilence again, which means you'll need a blood rune available at the proper time. (Thanks to a glyph, Pestilence will re-infect both the main target and all others in the group back to full duration on all diseases. Without the glyph, you would actually need to recast Plague Strike, Icy Touch, and Pestilence.)
- I have a ghoul up all the time (via talents). If he dies, I make sure to bring him back as soon as possible. He doesn't do a ton of damage, but every little bit helps.
- I have a trinket that gives me additional attack power for 20 seconds, and has a 2 minute cooldown. I try to use this as often as possible to increase my DPS output.
Now we get into other abilities that get used on an as-needed basis:
- I can Summon Gargoyle once every three minutes. I usually save him for bosses or other high-HP targets. To get the most out of the Gargoyle, I need to let my runic power build to 90 before casting.
- Death Grip: It is usually a bad idea to cast this in a raid, as it will immediately draw aggro. However it might be needed in very limited circumstances, such as to pull a caster in range of the tank, or if there is a mob attacking your healer.
- Army of the Dead: This normally has a 20 minute cooldown, though I have talents that lower it to 10. Even so, you definitely want to save it for special occasions. And there are certain boss encounters that will go haywire from AotD, since they taunt everything. But it can be useful in the right circumstances.
- Chains of Ice: Will stop a mob in its tracks for 5 seconds, with slowed movement for an additional 10 seconds. Doesn't do any damage (unless you have a particular glyph), so it is usually useless in a raid except in rare circumstances. Much more of a PvP skill.
- Mind Freeze, Strangulate: These can each prevent casting of spells for a short duration. Again I rarely use them in raids; I find them more useful in PvP.
In summary, playing a Death Knight is not for the lazy, stupid, or faint of heart. If you want to excel at playing this class, you really need to work at it. The problem is that since the class starts at level 55, it appeals specifically to the lazy and stupid. And any facerolling moron can lumber through all the quests in Outland and Northrend with no problem whatsoever. Its not until the Deathtard finds himself in an instance with a group for the first time that the stupidity becomes readily apparent... usually to everyone but the person playing the character. All he knows is the group wiped; he doesn't realize its because he kept pulling aggro off the tank and had the damage output of a band of level 20 gnolls.
For my benefit and the benefit of those around you, I implore you to STOP playing your Death Knight if you aren't willing to spend the time and effort to learn to play it correctly. Go back to your hunter or paladin instead.
And for God's sake, stop using Death Grip in raids, or I'll kill you myself.