Not posting exact numbers, but:
+5 drain skill
Doesn't seem to improve cures or damage. Furthermore, defensive and offensive seem to cure for the same and deal the same damage. The cooldown is reduced from 17 rounds to 11 though.
+5 thrust skill
Target has less chance for an extra attack.
Greater chance to stop target fleeing or speaking.
Damage in defensive and offensive is approx the same. I thought offensive used to have a bleeding effect I could be wrong.
+5 stance skill
Constitution increased in defensive stance.
Strength and Dexterity increased in offensive stance.
Strength, Dexterity and Constitution increased
in balanced stance.
Defensive stance gives 2con, offensive gives 1str 1dex, balanced gives 1 str 1 dex 1 con. These numbers used to be higher but likely never got 'fixed' after Hoss' nerf to rank stats. Still, shouldn't they be proportional?
Also, unholy might used to be +str, +hps... it doesn't do that anymore. Perhaps these skills/spells never got switched back to 'normal', but I think its worth taking a look at.
Dark blessing also has been changed quite a bit, it got nerfed as well as duration reduced.
Impunity has changed a lot, at one point it was hardbash at legendary. Now, at legendary, it seems useless.
Divine spells by DK are really hopeless. Perhaps moving summon dragon to 1st circle would allow dark blessing and unholy wrath to be less useless.
Lastly, a little transparency between stances would be nice. It appears that aside from the way skills work, there is no difference between the stances, only that Unholy Might casts differently between stances.
A little tweaking with Dark Knights would restore them to their former glory, as after their revamp they were quite a unique versatile class but are now lacking.