Death Grip is perhaps the most infamous of the death knight's abilities specifically because of how it can mess up a tank's day when misused. Remember, when you use Death Grip, you are taunting the mob you pull and it will attack you, forcing the tank to pull it off you and the healer to spend extra mana keeping you alive until that happens. When you hear people casting hate on death knights, a lot of time their anger comes from being grouped with a death knight who misused Death Grip.
So, the number one rule of being a good DPS group member as a death knight is probably in knowing how to use Death Grip. Here's a few rules.
The primary rule is pretty simple. Do not use it on cooldown or at random. It doesn't do any extra damage. It doesn't apply any type of useful debuff. Death Grip is a sometimes button. Keep it that way. Keeping this in mind solves most of your problems.
That said, there's still times when it can be useful. If you're facing a pull with lots of caster mobs, there's a good chance your tank will not have the tools to wrangle them all. That's when you say, "Hey. Heads up, tank, I'll go ahead and use Death Grip to bring that caster in so we can get it down in the AoE with the other mobs." Chances are your tank will appreciate both the heads up and the pull assist.
There's also keeping mobs off the healer or other DPS. It happens to the best of us. The tank is working on wrangling the main mob, but for whatever reason, they've lost track of that one that's eating the healer. At that point, you're in plate armor. You can probably peel the mob back to the main pack and survive a couple hits longer than the healer could. That's a point at which you can probably fire off an emergency death grip.
Ultimately, another way to solve the issue is to use Glyph of Tranquil Grip. Personally, I do not use this glyph because I like having the ability to have an emergency taunt as a DPS specifically to help save healers or wrangle a stray mob. However, if you're really having trouble with the misuse of Death Grip, you may want to invest in one anyway. Now, this doesn't completely shut down possible complaints. Some tanks will still get pretty annoyed that you pulled a mob out of position. Ultimately, your better bet is to just learn the proper use of Death Grip.
Army of the Dead
Army of the Dead is another skill that can make or break your reputation in a group. The biggest reason for it is that the ghouls can taunt by default. This means that when you summon them, a previously tranquil situation can turn into chaos. As the ghouls taunt mobs, they not only pull them off your tank, pet pathing AI means they drag them all around the area, thus increasing the chance they could pull in nearby adds. A poorly used Army of the Dead can lead to a wipe.
Thus, there's a few tips to using this skill effectively in a group. First and foremost, don't use it offhand on anything that can be taunted. Unfortunately, this does mean it's regulated to emergency use in most small group situations. This means you only use it if a wipe is imminent - that is, if the healer or tank is dead or near-death.
The easiest way to take care of this is to get the Glyph of Army of the Dead. While this removes its "emergency wipe saver" function, it means you can use it as part of your normal damage rotation with impunity. Ultimately, I'd say it's worth using a minor glyph slot for that peace of mind. Slot this glyph and pop Army of the Dead just before your tank pulls the boss, and you'll be a DPS superhero.
Don't get ahead of yourself
In non-class based issues, it's important to remember that tanks need to be able to keep aggro and set the pace, not just for their sakes, but for the sake of the healer who needs to stay alive and keep everyone healed. If you really want to "set the pace," I'll be frank: switch to blood spec and queue as a tank. Otherwise, stay back and let the tank handle it. Don't pull unless the tank asks you to, and make sure the tank has aggro before you unload your DPS. While sustained threat has been buffed to pretty high levels, tanks still need a bit of time to get initial aggro. So let the paladin get Consecrate down, let the druid get in a few Swipes, let the monk throw down a few barrels of brew. Don't be afraid to hold off on the Howling Blast spam for at least a few more seconds.
Don't let gear greed make you a jerk
Some people are still running dungeons for gear, whether it be because they can't stand Timeless Isle, or can't get that specific slot to drop. Be courteous to your fellow dungeon runners and try not to take gear they could otherwise use if it's not optimal for you as a DPS death knight. The most flagrant case for this is agility weapons and jewelry.
Agility weapons used to be at least somewhat passable for tanks, in that agility provided extra dodge. Agility weapons no longer do that. As a result, it's very likely that an agility weapon or ring, even if it's a higher item level, could be worse for because agility doesn't provide the attack power that strength does. In the end, rather than rolling on that agility polearm, you're better off letting the monk have it and doing some timeless isle until you can buy the strength weapon.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.