It's hard to offer good commentary on talents during a beta, simply because they're likely to change, sometimes repeatedly. Still, I love messing around with specs, Death Knights have a portal that leads straight to their trainers, and respecs cost a single copper to facilitate testing. As a result, I've taken most of the major talents for a spin, even if they're likely to change over time.
As I noted when I wrote up my first reactions to the class, Death Knights are required to use all three of their spell schools due to the rune system; you can swear off Frost if you really want to, but you'll be sitting on your tail waiting for your other runes to regenerate. This encourages a greater degree of hybrid specs than you might see in other classes. If you're going to have to use all your abilities, it makes sense to improve all of them. This is especially true with many abilities depending on the number of disease debuffs you've inflicted upon your target (often from a tree other than your main spec.
Here's what the trees have in them right now:
Blood: Heals. Lots and lots of heals. Blood spells also represent your bread and butter damage attacks if you're Unholy or Blood speced.
- Butchery, Rune Tap, and Vendetta: I group these talents together since they're all under 20 points and therefore available even if you're going pretty far down one of the other trees. Butchery generates runic power when you kill stuff and lets you passively earn more runic power while in combat. Rune Tap lets you use a blood rune to heal yourself for about 10% of your health, which is great for reducing downtime. Vendetta heals you when you kill stuff (which, in turn, stacks with other heals) and thus reduces your downtime even further. With apologies to Unholy Frost builds (if any), these talents are nigh essential for leveling.
-Abomination Strength: Gives you a moderately lengthy boost to strength every time you use Obliterate, a Frost attack that consumes all your diseases to do massive damage (but see the Frost tree below). It's a long enough buff to be usable in a pure blood build (i.e. you're probably finishing off the mob, but the buff will last through the next fight), but it really shines in a hybrid build with Frost.
-Bloodworms: This 3-point 30 point talent is the current pinacle of Blood tree healing. In its current form, it has a 30% chance to proc 3-5 worms each time you kill a diseased mob. This is hideously overpowered when it procs (and it can proc multiple times, I've had 8 worms up). The worms strike for 100-200 damage each every 2 seconds or so, and heal you for 100% of the damage they do. If you're fighting more than one mob, the worms will immediately spring into action and heal you to near full. Blizzard is changing this talent to be highly likely to produce a single worm from your Death Strike ability. This change will be interesting because it will let the DK choose between using those Unholy runes for more damage or for a reasonably good chance at self-heals.
- Heart Strike: No commentary on this one for now, because it's changing from the form it was in when last I tested it.
- Dancing Rune Weapon: You get a dancing rune weapon that greatly improves your damage... if you're fighting a stationary target that doesn't die. I'm sure this thing will be impressive in raids, but I didn't find much use for it solo; I kill stuff fast enough as is most times. Also, be warned that this thing will attack mobs you're not in combat with.
Frost: Crowd control. Frost has a lot of combo abilities that do additional damage to foes already afflicted with frost debuffs (like a frost mage).
-Annihilation: This talent effectively removes the major cost of the frost attack Obliterate. Untalented, the attack removes all your diseases from the target in exchange for large damage. Talented, the attack has a 90% chance not to remove the diseases, letting you spam it (and proc Abomination Strength, if you've got it, from the Blood Tree). This is a lot of damage, and even moreso if you use Deathchill, the 21 point frost ability, for a guaranteed crit. If you want to dual wield, you will be going this far down the frost tree anyway, so this is a no-brainer.
-Howling Blast: This is an AoE attack that currently does massive damage to frozen targets. Frost currently has 2 controllable freeze abilities (one a 51 point talent) and one proc, but the mechanic is being reworked down the line. This ability was a lot of fun the last time I took it for a spin, so I hope it remains so.
-Frost strike: The current 41 point frost talent does a small amount of damage that becomes large against frozen targets, but didn't impress me much since I already had Howling Blast for that. The new version will do more damage and not depend on frozen debuffs, which I think will be an improvement. Note, however, that at 41 points, this talent rules out the Bloodworms, and even Vendetta will not be available until the late 60's.
-Hungering Cold: I didn't test this thing much because sinking 51 points into Frost (and thus having almost none for Blood) made leveling painful. The spell is like an AOE hunter freezing trap, which made a lot of sense when Howling Blast and Frost Strike procced off of frozen targets and less sense now (it will still affect Howling Blast). I'm sure there are situations where this thing kicks some serious tail (if nothing else, at 100 runic power it lets you run ;)), but I don't see it being worth sinking this many points in the tree, especially now that frozen status doesn't matter for DK's.
Unholy: Pets and debuffs. If you go this route, you'll be inflicting more diseases on your foes and using the other two trees to take advantage of the additional diseases.
-Epidemic: This is a first tier talent that's moving down to tier two, and quite possibly a must-have. It extends the duration of your diseases, which means that you don't have to reapply them as often. That lets you focus your efforts on doing more damage.
-Corpse Explosion: Diablo II necromancers, rejoice. :) In its current form, Corpse Explosion consumes a lot of runic power (and, obviously, requires that your foes be standing near a corpse). The new form will consume an unholy rune, and will IMO be more usable as a result.
-Shadow of Death: This is possibly the most unique talent in the game. When you die, you come back as a ghoul for a limited time. You've got the ghoul's attacks instead of your own, but still, there's an amusement value for raising yourself from the dead. You will still get quest credit for anything you kill, but you'll need to make the corpse run to loot any quest items. (If you win the fight and want to start the corpse run now instead of waiting 30 seconds, click off the buff, or push the "explode" button.)
-Ravenous Dead, Ferocious Dead, and Master of Ghouls: I've grouped these talents together because they collectively all buff your raise dead spell. When the deed is done, the duration on your ghoul will be longer and the cooldown on the spell will be shorter, ensuring that you can always have a ghoul up at any given time (in fact, you can have two up at once for 90 seconds or so). Oh, and you get a freaking pet bar. That's right, this turns your ghoul from an uncontrollable DPS dump into a real live pet that does your bidding. He's got a spell interrupt, a stun, and another disease to boost the damage of Obliterate. This talent literally changes you into a melee pet class, and, at 26 points (27 if you also want Unholy Rune Mastery, which you might as well get for a single point), it's available with one of the other two 41-point talents.
-Summon Gargoyle: I didn't use this thing much when it sat at 41 points. At 21, it should become a nice fire-and-forget runic power dump. As always, beware of it going off to grab more enemies.
-Unholy Blight: An AoE disease generator. As with Hungering Cold, this limits your options, especially at lower levels, but I hear that there will be a substantial payoff for getting all of your diseases on a target at once.
If you haven't been able to tell, my favorite talents as of now are Bloodworms (31-33 Blood), Master of Ghouls (26 Unholy), and Howling Blast (31 Frost). The worms are a huge buff to healing, fully controllable ghouls are very very useful (and gives you the feeling of fighting side by side with your pet, slightly expendable though they may be), and the frost synergy reminds me of my Frost mage. Notice something in common? They're all low enough in their respective trees that you can have two of them at once before you hit Northrend. A level 80 characters can even have Howling Blast, Master of Ghouls AND still have enough points to make it as far down the blood tree as Rune Tap. If that's not a sign that Death Knights were meant to at least consider multi-tree specs, I don't know what is.
Cheerydeth, accompanied by a pair of ghouls, a netherwhelp, and an escort NPC. I actually felt bad for the groups of 2-3 mobs that spawned to try and stop us.