With all the upheaval regarding death knight abilities and DPS happening right now, it's sometimes a good idea to go back to the basics and figure out some of the stuff that underlies a good, solid death knight.
One of the most basic cornerstones of choosing a death knight is choosing your race. Since every single race can be a death knight, you have your pick, and since there's a race change service, you don't even have to stay the same race forever. Personally, I tend to say that you choose whatever race feels right for you -- but for those who want to pick a race (or change your current race) based on what racials are "best," this guide is for you.
Orc The biggest thing orcs bring to the table is the Blood Fury racial, which provides a nice little surge of attack power. Using it doesn't trigger the global cooldown, so you can even macro it into something else with ease. In addition, Axe Specialization provides some extra expertise for those times when an axe is a viable weapon choice, and Command will be useful for unholy DPS death knights by providing some extra damage for your ghoul pet. Finally, Hardiness, while not amazing, does provide a little bit of stun resistance. In short, orcs are probably one of the best races you can choose for PVE DPS purposes.
Tauren The best thing tauren bring to the table is Endurance, which increases their base health by 5%. Now that health isn't as dear as it used to be, this isn't as amazing a racial as it used to be, but it's still probably the best Horde tanking racial, as far as survivability goes. War Stomp can also be useful and sort of an emergency aggro-gainer, lifesaver, or escape tool. They also have Nature Resistance, which is highly situational and overwritten by Mark of the Wild, like all resistance racials. Cultivation can be useful if you plan to go down the herbalist/alchemist route. As you can see, tauren are one of the better choices you can make if you plan to be a tank death knight.
Troll Regeneration is, of course, the iconic racial passive for trolls, but we'll do more self-healing with a good Death Strike anyway. Berserking is a pretty decent DPS racial (and, like Blood Fury, is off the GCD), although we're already awash in haste as it is. Throwing and Bow Specialization, alas, do nothing for us, and Da Voodoo Shuffle, while better than nothing, isn't half as good as abilities that let you straight up break out of things. Beast Slaying is good, but situational. Overall, trolls are sort of a good, solid, middle-of-the-road option for just about everything.
Undead Will of the Forsaken continues to be the signature racial of the undead, making them still a solid choice if you plan on doing a lot of PVP. Shadow Resistance can also be situationally useful, but it's impossible to plan for using it. Cannibalize can be great for leveling, since it allows you to heal and move on more quickly, although again, Death Strike will probably take care of most of your self-healing needs. Underwater Breathing isn't as great as it used to be since Blizzard lengthened breathing time for everyone, but it may make a couple underwater quests less annoying.
Blood elf Arcane Torrent is certainly a nice way to gain some Runic Power on cooldown or interrupt, but we have so many interrupt options already that it probably won't provide as much of a PVP advantage as Will of the Forsaken. Arcane Resistance, like the other resistance racials, is nice but not incredibly useful. If you're planning to be an enchanter, you can pick up some extra help from the Arcane Affinity racial here. In short, blood elves are the race you're most likely to roll just because you like their style.
Goblin Time is Money is the big winner here, and it's enough to make goblins competitive with orcs for the title of best Horde PVE DPS race, especially since you don't have to remember to activate it like you do with Blood Fury. Since it appears to be 1% of real haste, it's a great buff for at least two of the three death knight PVE DPS trees. Best Deals Anywhere and Pack Hobgoblin are great time- and money-savers out in the field, and Better Living Through Chemistry gives you a jump on Alchemy if that's your chosen profession. Rocket Jump can be useful for escaping sticky situations or closing in on an enemy in PVE or PVP. Rocket Barrage, alas, is on the GCD and thus is not really as useful as it could be. Essentially, you may find some use for this for pulling or for adding some extra DPS when you can't be in melee range of your target, but that's about it. It also shares a cooldown with Rocket Jump, so keep that in mind when you use it as well.
Human Every Man For Himself is the human's claim to fame, and it makes them dominant as a PVP choice. Even in PVE, it can be a lifesaver to break out of certain boss mechanics, though, so don't underestimate it. Humans also get Sword and Mace specialization, which means you have a pretty decent chance of being able to grab some free expertise on your weapons, something that DPS death knights should love. Diplomacy won't give you an edge in battle, but if you're aiming for The Exalted or just want shorter grinds on all of the required raiding/gearing-up factions, humanity is hard to beat. The Human Spirit is the last racial, which, alas, offers no real benefit to speak of for us death knights.
Night elf Quickness is a pretty solid tanking racial for death knights, and Shadowmeld may help you survive while DPSing if you're a bit overzealous -- just hit it and remain hidden until the tank has some solid aggro again. Wisp Spirit is slightly useful some someone who doesn't have much luck surviving, and Nature Resistance is the token resistance racial.
Dwarf Stoneform is a rather nice racial, enough to make dwarves arguably the preferred tanking race for the Alliance. It's most useful for PVE tanking, allowing you to take even more punishment just when you need it. DPSers may find it useful for powering through AOE attacks or removing debuffs, though, and it even may be what the doctor ordered for getting you out of a rogue's stunning attacks in PVP if Icebound Fortitude is down. Mace Specialization may end up coming in handy, depending on what you're wielding. Gun specialization, alas, is useless to us, and Frost Resistance carries the usual resistance ability disclaimers. Finally, Explorer, while not directly useful for fighting, may make it easier for non-raiders to dig up the ilevel 359 gear they want out of the skill.
Gnome Escape Artist has some limited use in PVP and even PVE, but it's not as good as the human racial, of course. Shortblade Specialization has limited usefulness for dual-wielding frost death knights should they manage to pick up a couple swords. Arcane Resistance is the token resistance racial here, and Engineering Specialization offers a shortcut on what is one of the death knight's most competitive professions for endgame use. Finally, Expansive Mind is pretty much useless for us, as we have no mana and it doesn't similarly boost our runic power pool. Gnomes, like blood elves, are the race you roll primarily because you like their style.
Draenei Heroic Presence is your main racial motivation for rolling a draenei, and it's a pretty good reason. It gives you some extra flexibility in gearing, since you'll only need to hit 7% hit from gear. Gift of the Naaru can also be incredibly useful for an emergency tank button, especially since it's off the GCD and thus can be macroed and/or activated without breaking an existing rotation. Other than that, Gemcutting offers a head start on a great profession, and Shadow Resistance offers limited protection against select mobs.
Worgen Viciousness is the worgen racial, and while not every DPS spec is crazy over critical strike chance, this still makes worgen a strong contender against draenei for best PVE DPS Alliance race. Darkflight is another awesome racial that may even push them past draenei for best PVE race. It's great for getting away or chasing down runners in PVP, and it's awesome for getting out of the bad stuff in PVE. Other than that, worgen get a bonus to their Skinning, which is a very lackluster profession for death knights, and Aberration, which is as lackluster as any other racial resistance ability, but does at least cover two resistances to everyone else's one.
The final verdict on death knight races
If you just want the quick and dirty on what to roll and you're only concerned with in-combat performance, here it is: If you want to tank, roll a dwarf or a tauren. If you want to DPS, roll a worgen, a draenei, a goblin, or an orc. If you want to PVP, roll a human, a worgen, or an undead.
But I'm just going to reiterate this again: You can make any race work, and no race is so overpowered that you should feel it's mandatory to pick it. Play what makes you feel awesome, and you'll be awesome. Just remember, your racials can offer some help, too. Don't leave them languishing in your spell book.
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.