From a tank perspective:
New Ability - Unholy Blight
40 Runic Power, 30 sec cooldown
Surrounds the Death Knight with a vile swarm of unholy insects for 10 sec, causing X shadow damage every sec to nearby enemies within 15 yards. (Damage range similar to death and decay)
New 1st Tier Talent - Infestation (replaces unholy blight)
Your Death and Decay and Unholy Blight now apply Blood Plague and Frost Fever to all affected targets.
New Ability - Blood Link (Blood Only)
1 Blood, 20 yard range, 3 min cooldown
Links to the target, causing 50% of damage taken by either player to be transferred to the other linked player. Lasts for 12 sec, players must remain with 20 yards of each other or the link will break.
New Ability - Blood Mirror (Blood Only)
1 Blood, Channeled, 3 min cooldown
Links to all nearby allies within 30 yards, causing 20% of all damage taken to be transferred to the death knight. Transferred damage is taken over 10 seconds once Blood Mirror ends, but cannot be absorbed or reduced. Players who move more than 30 yards away will break the link. Lasts 6 sec.