Because we start at level 55 and unlock our talent points over the course of only three levels' worth of quests, the question of which tree to put your talent points in comes up much sooner for Death Knights than any other class. We are also unusual for having talent trees that are less clearly-defined than most classes, as you can find a spec in any tree for either tanking or DPS (though you shouldn't rely on the same one for both if you want to do either well). There are more nuances than just, "This tree is for tanking," and as a result your choice of spec is based more on your own personal preference than anything else. So which one do you pick?
Before I really get into the meat of it I'd like to note that this post isn't going to be about min-maxing except for the occasional "As of this posting..." comment. We didn't see nearly as many adjustments to our class in the most recent patch as we have in prior ones, but Blizzard is still working out the kinks. The flavor of this month may not be the flavor of next, and I would like this post to still be a useful resource next month. Therefore, I'm just covering the concepts of the different talent trees rather than what's currently best. As always, I recommend Elitist Jerks for up-to-the-minute theorycrafting.
I'll start with Frost because, as you may have noticed, it's my specialty. Early in development, Blizzard declared Frost to be the tanking spec for Death Knights. They later changed their minds and gave all specs viable tanking options, but it's kept that reputation. More recently it's become the dual-wield spec because it is the only tree with talents that apply specifically to one-handed weapons. Both of these are misconceptions to a certain degree: It has excellent, solid tanking options but is not the only tree for the task, and while your DPS will be gimped if you try to dual wield as any other spec there are perfectly good two-hander-based Frost specs.
Personally, I see Frost as the AoE spec, whether you're DPSing or tanking, no matter what weapon you're carrying. The tree's 51-point talent, Howling Blast, is an AoE powerhouse for both damage and threat, doing damage on par with Obliterate on each mob it hits. Frost tanking is comparable to Paladin tanking for AoE threat generation, and it's excellent in any situation with trash pulls, especially if you're dealing with a rowdy PuG. For leveling, it's best for burning down large groups of mobs. If you're going to dual wield, the talents Nerves of Cold Steel and Threat of Thassarian are absolutely required for reasons that are self-evident in their descriptions. No other tree has talents that are specifically geared to bringing DW damage in line with two-handers.
The downside to Frost for most people is that, compared to the other trees, it's frankly kind of boring. There's no real gimmick to it. Even from a lore perspective, you have the necromancy tree, the vampirism tree, and the...hypothermia tree. The 9k+ AoE crits keep it interesting enough for me, though. The emphasis on proc-based talents like Killing Machine and Rime make it more reactive, which doesn't appeal to everyone.
TL;DR: Spec Frost if you want to dual wield or if you want to have strong AoE capabilities.
The whole point of Blood is survivability, both through raw stamina and self-healing. Early on it was primarily considered a leveling tree, since the self-heals meant you could keep going with very little downtime. More recently it's become a popular raid-tanking spec, trading Frost's AoE threat for talents that just plain keep you alive longer. Where Frost's threat generation is like a Paladin's, Blood's is like a Warrior's, with more emphasis on single targets. DPS is likewise more single-target based, with Strength-buffing talents that improve your overall damage but no AoEs except the basic class ones. This is fine for leveling, since you likely won't have more than a couple mobs to deal with and you can survive long enough to take them all down one at a time. It's also very good for boss fights. As a whole, Blood is the least magic-focused of the talent trees.
The downside to Blood is that when tanking, much like Warriors, you need time to ramp up threat on multiple mobs, which requires your DPS to actually show some restraint and maybe follow a *gasp* kill order. For DPS, your numbers won't look quite as good on those big trash pulls as more AoE-based classes.
TL;DR: Spec Blood if you want self-healing and extra survivability.
The poor Unholy tree has been nerfed and buffed and tweaked all over the place since Wrath went live, but the basic concept has stayed the same: This is essentially the pet spec. All Death Knights can summon a ghoul for a short period, but with Master of Ghouls you get a permanent buddy. It even gets a new name randomly generated from some ghoulish words every time you raise one, some of which are unintentionally hilarious: I have a friend who once summoned a Gravelstealer, inspiring us to express concern about the safety of our driveways, and I once saw a Bonegobbler which made me giggle because I have a dirty, dirty mind. The perma-ghoul is a great little DPS boost for groups or soloing as well as an irritant for PvP. It's also useful for distracting any mobs that might attack when you're mining or herbing. I went Unholy DPS for a bit before 3.2 made Frost dual wielding viable, and I missed my ghoul when I changed. Unholy also has an AoE in the form of Corpse Explosion, but it requires you to have finished killing something to use it, so it doesn't have quite as much utility as Howling Blast.
The downside to Unholy is that, while you certainly could tank in an Unholy spec, there's nothing to recommend it like Frost's AoE threat or Blood's survivability. There's also the added responsibility that comes with using a pet, such as keeping it from randomly attacking the wrong things.
TL;DR: Spec Unholy if you want to play as a pet class, or if you think making corpses explode is the best thing ever.
Hopefully now you have a better idea of which talent tree is right for you and your up-and-coming Death Knight. In an upcoming post I'll get more in depth on the nuances that can make the same tree good for both tanking and DPS roles!