Well, I've had a bit of time to put the Death Knight through the paces, so I figured I might as well post my impressions so far. I cleared out almost every quest in Hellfire Peninsula (starting at level 58, leaving almost done with 61) and have messed around with a few spec variations. Obviously, opinions may change depending on how things scale, etc, going forward.
Lore: What is it good for?
The new DK starting area is definitely well done from the lore perspective. There are some spawns that will probably be hard to get on launch day, and some issues with some quests that use the new vehicle combat mechanics. (Broadly speaking, in the rest of WoW your attacks hit as long as you're in range and your target is within your 180 degree cone of vision, whereas vehicles seem to require that you actually aim correctly. This is fine, but learning it under pressure can be a pain.) It definitely sets the stage for one of the plotlines in Northrend, as well as explaining where all these Death Knights flooding your cities came from. At the moment, the game doesn't do a very good job of transitioning you from the introductory questline into the real world (much less explaining why a 10 level detour to Outland before going after Arthas makes sense), but we're told this will be fixed later in beta. Overall, the lore is good. Rohan recommends that everyone try the starting area, and I agree, though you might be better off waiting a week or two for the rush to die down a bit.
General Power Level: High
So I spend a fair amount of my time running around doing daily quests, so I have some experience tearing through mobs with the benefit of better gear. There's simply no comparison to what the Death Knight can do to mobs at least as far as I've gotten so far. Again, part of this may be due to gear; the DK will get very few upgrades from outdoor quests in Hellfire Peninsula. Overall, though, this is by far the easiest romp though Outland I've had so far, even compared to my T2 epic mage (with practice in the TBC beta). I can solo tough elites and bosses, sometimes several levels above me. Crowds are not a problem because I can do some AoE damage, perhaps unleash a minion, and burn down one of the foes (which starts healing me enough to make it through the rest of the pack). Caster mobs are a cakewalk, since I have a skill that physically pulls them to me (stopping their spellcast), a pummel-like interrupt, a silence, and potentially a pet spell (for Unholy-speced Ghouls). I'm not sure if some of this has nerfs coming, but it's pretty impressive to see.
Double Edged Sword
To the extent that the DK has a weakness so far, it may be lack of control. I have massive amounts of self-healing potential, but the of these are proc dependent. (If the worms proc, you win.) Depending on spec, I have a wide variety of minions at my disposal, but none of them are fully controllable. (If you're willing to spec 26 points into Unholy, the default Ghoul gains a pet bar that allows you to direct him on what to kill, or detonate him if you tire of his antics. However, at the moment it does not appear to allow you to tell him to come back to you before he aggros more mobs, or to stay put/passive/etc. Seriously, PUG's are going to have a lot of trouble with DK's who can't control their pets.) Which brings us to...
The Rune System
In case you don't know, the Death Knight has the usual health bar, a runic power bar (where energy/mana/rage would be) and six runes. Runic Power is pretty straightforward. You get power for using abilities (kind of like warrior rage, but more emphasis on use of abilities rather than dealing/taking damage, and it does not decay anywhere near as fast out of combat), and use it to cast runic power abilities. The actual runes, though, are a novel and interesting mechanic.
You've got 2 blood runes, 2 unholy runes, and 2 frost runes, which you use to pay for abilities. Runes recharge automatically in something like 10 seconds after using them, kind of like a Rogue's energy bar. There are some talents that allow runes to regenerate faster (again, generally via a proc rather than something you can click at will to regenerate), but, in general, this means that you have to use different spells as you wait for runes to recharge.
For a typical combat example:
-Pull with Icy Touch (instant cast, 20 yd range, 1 frost rune), which does some damage and slows enemy attacks.
-Hit the enemy with Plague Strike (instant cast, 1 unholy rune and one blood rune), which does damage and inflicts a disease.
-Hit the enemy with Blood Strike (instant, 1 blood rune), which does extra damage to diseased targets.
-Now you're out of Blood runes (if a talent procs getting you more blood runes back sooner, you can spam blood strike). Depending on my mood, I might hit another Icy Touch, simply to burn a frost rune I'm not using, or perhaps a Death Strike (1 unholy, if the mob dies within 5 seconds I get healed) to get some runic power. This step is optional if you had some runic power leftover when you started the fight.
-At this point, presuming I was starting from zero, I have enough runic power to launch a Death Coil (instant cast, 40 runic power, does damage). Spam Death Coil until out of Runic power.
-Presuming you haven't taken points in Unholy, you have 2 seconds between the time when your first blood rune (from the Plague strike) regenerates and when the disease from the first Plague strike wears off. This allows you to make another blood strike. At this point, a non-elite mob is probably dead. Otherwise, you can refresh Plague Strike and start over.
What's notable about all of that is the number of spells you actually use. I don't even have all of the default skills for DK's yet, and I'm already using half again as many different spells per fight than my live characters (indeed, the mage often gets by on Frostbolt alone). This also means that hybrid specs are more attractive than they've been for other classes; you're going to need to cast some of each type of spell, so it can be worth improving a wider range of them than usual. But this post is getting a bit long, so I'll save spec discussion for another day.