This looks like, overall, it will be a solid damage increase for death knights. The two biggest questions are how often it procs, and how it will work for dual wielders. It seems likely they will apply it to both weapons, but if they don't, dual wield frost could suffer for it this patch. It's also worth noting that unholy will get more weapon use out of this than is usual for the spec, but it seems doubtful it will be so much as to massively change the way they gear up.
Endurance of Niuzao ends up being kind of a pseudo-Purgatory, except a bit better because it completely prevents the attack rather than creating a health debt. We assume both stack, of course, so this basically means death knight tanks get a couple extra lives depending on their talent choices. You'll still need a few heals to prevent yourself from immediately dying anyway, but death knights are in a special place to take advantage of that thanks to Death Strike and our other self heals.
Blood Death Knights
Item - Death Knight T16 Blood 2P Bonus - Every 4 Heart Strikes, Rune Strikes, or Blood Boils will add one charge to your next Bone Shield.
Item - Death Knight T16 Blood 4P Bonus - Dancing Rune Weapon will reactivate all Frost and Unholy runes as Death Runes, and make your next 4 Death Strikes free.
Blood death knights get some excellent bonuses this tier that offer some fun new ways to play. The two piece bonus is especially interesting because it specifically adds the charges to your next Bone Shield, not your current one. This means that Bone Shield becomes less of an always-on thing and more something you deliberately save for when you need an extra burst of avoidance. You can also look at it as an opportunity to build up charges on trash and have an extra-beefy Bone Shield when it's time to face the bosses.
The 4-piece bonus not only acts like a souped-up Empower Rune Weapon, but it gives you free Death Strikes. This is not only great for surviving damage spikes like never before, it gives you the freedom to unleash even more burst damage or threat with the death runes. Having trouble keeping a group of adds under control? Hit Dancing Rune Weapon and use those death runes on Blood Boil. Plus, your Dancing Rune Weapon will be up during this to do even more damage. Blood death knights are definitely going to want to gear toward their 4 piece bonus this patch if it stays how it is.
DPS set bonuses
Item - Death Knight T16 DPS 2P Bonus - Killing Machine and Sudden Doom grant 500 Haste or Mastery, whichever is highest, for ($hnd * 2 + 4) sec, stacking up to 10 times.
Item - Death Knight T16 DPS 4P Bonus - Death Coil increases the duration of Dark Transformation by 1.5 sec per cast. Pillar of Frost increases rune regeneration speed by 50% while active.
Alas, DPS bonuses are a little more lackluster, though to be fair, it's hard to top summoning your own personal army. Still, the DK community's done the math, and even the math's not amazing for these. The two piece bonus leads to an even larger overload of haste than we already have, and there's some death knights thinking it may simply be too much. Since we don't have any indication of what "hnd" means, we can't do solid math on this yet until we know how long the up-time really is. "Stacking up to 10 times" would seem to imply to me that it's meant to be up most of the time, however.
The 4 piece bonus has been a subject of hot debate, especially in the differences between the specs. The recent nerf of the Pillar of Frost regen speed is more or less to be expected. 100% regen speed appeared to be leading to GCD capping, with frost making more resources than they could spend. 50%, then, is not as big a nerf as it first appears, as we couldn't use the resources anyway.
On the other hand, the 4 piece bonus for unholy has been nearly universally panned, at least in terms of math. You won't be putting out death coils fast enough to keep Dark Transformation up indefinitely, and so far, math suggests you're only getting another 20 seconds of uptime with perfect execution, with total extra damage only reaching around 0.5-1%, which is certainly underwhelming to say the least.
Another argument some unholy death knights have made is that it takes away some of the more compelling mechanics for unholy death knights by nearly diminishing the Death Coil to Dark Transformation buildup. Of course, it's fair to point out that other death knights have argued for the removal (or diminishing) of that building in the past, so this may be a good example of why Blizzard's never caved in and dropped Shadow Infusion down to 3 stacks, for example.
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.