Blood gets deadlier
Blood tanks got no major changes in the patch, and yet their issues come up often enough in forum threads and blue posts. We've even discussed them here. Essentially, the whole "active block" principle of Death Strike, while intriguing in theory, has lead to a rather frustrating style of play in which you must maximize your used of Death Strikes by any means possible, eschewing the use of your runes for abilities that might give you better threat, damage, or utility.
This style of play is still going strong in patch 4.2, but at the same time, tank death knights have received the one major death knight-oriented hot fix of the patch so far, as Improved Death Strike got a huge boost to damage output potential. This is because death knight tank single-target damage was just too low for certain Firelands fights, such as Alysrazor. Believe it or not, the amount of damage you put out as a tank is pretty important. It not only provides better threat, but it means you get the boss down faster. Death knights have been at the low end of that totem pole in raid boss situations for a while, in part because we're using our runes on Death Strike instead of Heart Strike.
This change has caused a bit of grumbling among some in the death knight tanking community. There's concern that the fact that Blizzard buffed Death Strike means the devs have given up on us, that they've thrown in the towel and just decided to let Death Strike be the primary strike, buffed as needed to keep us competitive. I'd argue it does nothing of the sort. What it does tell us is that Blizzard is aware of what Death Strike is for us right now. The devs can't really rebalance the whole spec in a hotfix, so they're doing the realistic thing and buffing the strike we feel compelled to use. If they had buffed Heart Strike instead, it would have been a sign they don't know what they're doing.
But with the buff to Death Strike, it's a pragmatic nod to the fact that it is what we need to use -- and, I'd wager, a herald for possible changes in patch 4.3, if not a theoretical patch 4.2.1. This hotfix proves that Blizzard is aware of what is going on, and with any luck, the devs'll make changes to fix it in a proper patch, when they have more time to test out the balance quirks.
Frost still freezes out unholy
It seemed as if everyone expected some pretty quick hotfixes to death knight DPS once patch 4.2 went live and we figured out how to break things. Instead, Blizzard has left well enough alone, and we're all learning to get used to the new DPS normal (at least until the next big balance patch). As expected, dual wielding is the order of the day for the frost death knight. Thanks to the MOTFW nerf and the Killing Machine buff, as long as you can get two weapons of the right ilevel, you will beat out 2H frost in just about every arena. The two most surprising things about frost are that even after the nerfs, dual wield frost can still come awfully close to patch 4.1 2H frost DPS levels, and it still beats out unholy.
Unholy mostly suffers from bad scaling and bad itemization. The best trinkets for an unholy death knight continue to be last tier's heroic trinkets, as the current strength DPS trinkets for Firelands rely way too much on critical strikes. That said, unholy has certainly seen a DPS jump in the patch and is viable. It's just going to be lower than frost. Patch 4.3 will probably still need to see some buffs to actual strikes (in order to facilitate better scaling with weapon damage) and possibly the use of critical strike chance by the ghoul pet to make that stat more desirable.
Since I have seen a lot of confusion from some commenters on the state of expertise for unholy DPS, I thought I should clear that up here as well. Expertise is, in fact, undesirable for unholy death knights. While it is slightly better than critical strike rating up to the 26 expertise soft cap, haste and mastery rating will almost always work better. You should reforge any expertise you get to the 8% hit cap first, then haste, then mastery. This is because unholy does a great amount of its damage with Death Coil, diseases, the ghoul, and Death and Decay. All of these abilities do not use expertise at all. Even your ghoul will gain what expertise it needs based on your hit chance. Therefore, with such a small proportion of your damage based on any sort of attack that uses expertise, it's better to reforge it to other stats and just deal with the occasional dodged Scourge Strike.
One other DPS question I've seen a lot is the usefulness of our two-piece tier 12 bonus. A little bit of extra runic power on Horn of Winter does seem at first glance to be a little underwhelming, but surprisingly, it works. Don't be afraid to break your four-piece tier 11 bonus to grab it. Initial parses have shown it to be a relatively adequate boost, in addition to the extra strength and other stats on the armor itself. It's still not as awesome as say, burning imps or flaming arrows, but I guess not everyone can have completely cool tier bonuses.
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.