Dark Knight is nothing like it should be; its really just a copy of Paladin with some AOEs and a half baked mechanic thrown into it. Should be about Life Manipulation; shouldn't even have a mana bar honestly. Just a bigger life pool and abilities that take life to use; and lifetaps.
Honestly though with Paladin changes they do 8 times more healing then Dark Knight can; and in Sword Oath they have near the same DPS. (Dark Knight does way more aoe dps though.)
I loved the "Dark" class in RIFT who was all about health manipulation.