The official PTR patch notes are always available at http://www.worldofwarcraft.com/patchnotes/test-realm-patchnotes.html. The list of Death Knight changes is extensive and there is no way I could describe them all in detail, so I am going to pick out the highlights and talk about what they mean for the class as a whole.
Pestilence - Once the bread and butter for quickening up pick AoE aggro on new mobs while Death and Decay is on cooldown, this function is being relegated to Blood Boil. Pestilence will now only spread the diseases on the Death Knight's current target, not inflict any immediate damage.
Blood Boil - Blood Boil has been adjusted to compensate for the change in Pestilence. It will now deal damage to targets that are not afflicted by diseases. It deals increased damage to diseased targets. Blood Boil's range has been reduced, and its base damage increased.
Blood Plague and Frost Fever - These abilities have had their base duration increased to 15 seconds, from 12.
Plague Strike - This ability no longer removes a HoT effect from the target. To compensate, its damage has been substantially increased.
Auras - Blood Aura, Frost Aura, and Unholy Aura have been replaced with Improved Blood/Frost/Unholy Presence. Specing into this talent adds a bonus to the presence, as well as allows the Death Knight to keep a function of each presence even while in difference presences. ie: Improved Frost Presence grants Frost Presence's health bonus in all Presences.
Sudden Doom - The Sudden Doom talent in the Blood tree now procs a free Death Coil instead of requiring you to push the button.
Howling Blast - Howling Blast: Swapped positions with Hungering Cold in the talent tree, and damage bonus to targets afflicted by Frost Fever reduced from 100% to 50%. Hungering cold is the 51 pt talent in the frost tree. Moving it up certainly justifies the damage decrease, but what most 2-handed weapon loving Death Knights don't know is that this change effectively destroys Death Knight dual-wielding as we know it. Currently, the only reason dual-wielding Death Knights can keep up on damage is because their talent build is spell based, and involves spamming Icy Touch so that Rime can proc a free Howling Blast. Blood Strikes with the Reaping talent (from Unholy) make it possible to chain up to four Icy Touches. With Howling Blast moving to the bottom of the tree, however, Death Knights will no longer be able to reach Howling Blast and Reaping, therefore completely destroying this spec.
Necrosis: Can now only be triggered by main hand attacks. Adding insult to injury, Dual-Wielding Death Knights are losing out on damage from this ability as well.
Scourge Strike: This ability has had its damage increased. Additionally, the Outbreak talent now further increases this abilities damage by 13/30/45%.
Gargoyle and Unholy Blight: Unholy Blight and Gargoyle have swapped places in the Unholy tree. Gargoyle is the new Unholy 51 talent. Reportedly, the cost of Gargoyle has been reduced, along with having its damage and duration increased.
