To start with the tanking nerfs, all of our major talented cooldowns were increased from 1 minute to 2 minutes. To add (some say) insult to injury, we also got a few more general nerfs in the form of Blade Barrier becoming a straight damage reduction (Which is in turn a nerf to Spell Deflection and Bone Shield, which stays up longer if an attack is parried instead of removing a bone) and Frost Presence losing some base magic damage reduction. Finally, Lichborne lost its 25% miss chance
To understand why Blizzard did this, you should probably start with this PTR thread from before this round of nerfs. In this thread, various tank classes and specs went up against the test Patchwerk at the entrance to Ulduar and recorded their survival time with various healer setups. As you'll notice, Death Knights simply lasted the longest, in most cases by multiple minutes.
Now, I wouldn't say that this thread was the only factor in Death Knights getting nerfed. I have no doubt that Blizzard ran their own internal tests that came across similar findings, and while you can always say player skill or a certain talent setup or gear level plays into it, looking at the whole picture, it's difficult to deny that our cooldowns aren't amazing. I know that when I am in trouble in a 5 man, as a basic Unholy tank build, I can hit my defensive cooldowns, switch to Death Strikes for my Frost/Unholy rune attack, and live through almost anything as long as my healer is doing something other than standing around picking his nose.
The trouble with this is obvious to start, but one other thing that was happening is that with multiple amazing powerful cooldowns, Death Knights could chain cooldowns and remain relatively invincible for long stretches of time. Hit Bone Shield. Bone Shield down? Use Icebound Fortitude? Once Icebound Fortitude fades, use Lichborne. With enough avoidance, Bone Shield will be up long enough for Icebound Fortitude and Lichborne to fill the gap until it comes back again. It's a bit harder to cover the gaps left when Unbreakable Armor and Vampiric Blood are on cooldown, but you can still have at least one ability up and running most of the time.
In part, this was sort of our expected way of tanking. We'd sort of assumed our niche was avoidance and cooldown tanking. The fact that Frost Presence was actually sort of weak on its own helped this along. Blizzard has already sort of tried to nudge us to be less cooldown dependent. All three talented cooldown tanking abilities have seen nerfs before now, with a nudge to Frost Presence to compensate.
It may be that Blizzard was at one point hoping to keep us on that interesting little cooldown dependent path, but it just became obvious that Death Knights were getting a bit too powerful. In addition, it brings up the problem of what happens if you miss a single cooldown and wipe.
Overall, we probably haven't lost too much of our raw cooldown power. Some say Unbreakable Armor still needs some work, and it probably depends on how well that new mechanic actually decreases physical damage. But Bone Shield and Vampiric Blood remain basically unchanged, and we should still be able to use our cooldowns to good effect when they're actually needed. Plus, now that Lichborne is mostly a PvP talent (Especially for Undead and Human tanks), Blood and Unholy tanks can feel like they don't absolutely need to waste so many points in the Frost tree.
Hopefully, we'll see more solid tank testing numbers soon, but I doubt the sky is falling for PvE Tanks.
Good news for some Death Knights, as it looks like dual wielding may be on the path to relevance again with the denerfs of Necrosis and Blood-Caked Blade. Howling Blast got a denerf too, damage-wise, but its still at 51 talent points and on a 10 second cooldown.
The bright side on this is that it opens the door for other possible Dual Wield builds. A 51 point Unholy Build that takes advantage of a fully buffed Ghoul and a Gargoyle is a distinct possibility, and in fact seems to put out some decent DPS according to various testers.
Of course, most dual wielders are probably used to Frost, at least mechanics and rotation wise, and luckily, Frost DPS still seems viable for dual wielders. 51 frost/20 Unholy is actually very viable in preliminary DPS tests, if not slightly superior to the above Unholy build.
I'm still not completely sure if 2handers and dual wielding can be balanced on the same class, and if one or the other has to go, I'd honestly prefer it to be dual wielding. However, Blizzard may finally be on the path to keeping the two in line with each other. We'll see how things change as the PTR Progresses, but at the least, I'd expect the Necrosis and Blood Caked Blade buffs to stay. Dual Wield balance probably hinges around them.
And the Rest
Despite the cooldown removal, Ghoul Frenzy still feels like it needs some tweaking, for the same reasons I've mentioned before. That extra Unholy rune continues to sit there messing up Unholy rotations pretty hardcore. In theory, you could save it for a Death Rune, but even then, that leaves you with one spare Death Rune with no real use.
In the end, I'll sign off with my usual notes of caution. We're a new class. We'll be going through some growing pains more than any other right now for sure. 3.1 was always going to be a pretty major patch for us, at least for anyone with a bit of foresight or understanding of how MMOs work. And it is still a PTR. Things can, and will change. Again, if you're concerned about these nerfs, I'll give my usual recommendation. Find solid numbers to support yourself. Go on the test server and do some tank runs and post recount and WWS numbers. Get involved and prove your concerns, to yourself and to others. This is your chance to help shape the next stage of Death Knight evolution. Go out there and grab it.