Aspects of the Scourge that Death Knights Haven't Touched:
-Ghosts/Spectres
-Plague Mushrooms/Necrotic Fungi
-Plague Cauldrons
-Gas/Blight (Putricide)
-Vermin (Rats, etc.)
-Poison
-Spiders (Both Maexxna and Nerubians)
-Reanimating things multiple times (ie, Black Knight)
-Liches (inb4 "but 'Lichborne's' naaame!")
etc.
Aspects of the Scourge that were Touched on by Death Knights but Have Been Cut:
-Insects/Locusts
-Sacrificing Minions to Heal
-Utilization of Blighted Ground
-Corpse Explosion (inb4 "muh Legion cosmetic")
-Self-reanimation
etc.
Aspects of the Scourge that are Currently Touched on by Death Knights but Underutilized:
-Plague/Disease (Unholy went from 7 DoTs to 1)
-Bones (There's what? One defensive blood passive now, and one offensive talent?)
-Mass Necromancy (Unholy has AotD, but no interaction)
-Slime (Unholy currently has a single PvP Talent which draws from this)