Dark Knight's Darkside move now constantly drains health. All skills now cost health instead of Mana. All Mana regens become Health regens instead. I won't talk numbers, since it would all change.
The issue with this method is that Dark Knight instantly becomes the straight up weakest Tank. Granted, it could be modified to make Darkside a DPS stance that you'd only ideally use as the Off-Tank for a high-risk/high-reward damage-dealing role. Which sounds cool! Sure, let's do that... but then DRK becomes the only Tank Job which cannot realistically utilize their OT stance while tanking. Sword Oath only loses 20% mitigation. Deliverance loses 25% health. Darkside loses 20% mitigation AND causes you to leak health on top of the damage coming in.
Hands-down, this method would decrease Dark Knights currently comparable DPS to a level that would essentially make it outmoded by Paladin. If they upped potencies across the board to account for it, you would have a massive skill gap between normal Dark Knights and the top-end Dark Knights who have healers babysitting them to maintain their drastically terrible Health management.
See - Darkside works as it is because
it is an independent state of being. No one on the outside can impact how much Mana you have. It's
your resource to manage. You need to take a hit to your damage - and a MAJOR one - to restore Mana mid-fight from an outside source (except during downtime). Switching this to Health beat-for-beat does one of two things:
A) It makes your Health your new resource, thus NO outside heals can help you while under the effect of Darkside. Suddenly you will either be THE WORST TANK FULLSTOP or your self-heals will be so powerful that you won't need a healer healing you - thus full-time DPS from both healers outside of raid damage. #broken
B) Your offensive and defensive power lives and dies by the skill of your healer. Your health drops and you're only saved by the healer's good graces. Now, people complain enough about Living Dead being worthless because healer skill in DF is below average. Imagine applying that to EVERY SINGLE TIME you press Dark Arts and chunk your own health, thinking you'll be fine, only to be slapped down to near-death, healer is slow OOPS you died. This sort of scenario would be common under this assumption/change.
I have more points to make, but I'll leave it at this, for now.