I am a 50 WHM and all week have been progressing through endgame content and along with my (also WHM) healing partner we have noticed the enmity generation from regen seems to be entirely disproportionate.
The issue is that rather than a flat enmity amount per cast of the spell, regen is considered almost exactly the same as any general cure spell and accruing enmity as such. (70 + HoT x 0.5 according to Japanese enmity tests)
As a basic example.
Going into the Amdapor demon wall boss fight I pre-cast regen on myself and my party members. assuming an average per tic amount of 350HP -This is a conservative number especially as regen tics can proc- on 4 players, regen is 'healing' 1400HP every 3 seconds.
Current enmity generation for cure is 0.5 for every point of cure, thus 1400 HP every 3 seconds means WHM enmity generation of 700 every 3 seconds before any active healing spells are cast. Overhealing -curing while the HP bar is full- generates enmity at the same rate as if the cure actively regenerates HP.
Obviously in endgame instances regen alone is not enough and further healing is required in addition to the regen. As a cure 2 can crit to in excess of 2000 a WHM can feasibly gain 1700 enmity in 3 seconds. (1000 enmity from a cure 2 + 700 enmity from the 'passive' healing from the regen tic)
To put this into perspective a 50 PLD using Flash while under the effect of shield oath only generates 1100 enmity, a deficit in the above case of 600 from the whm in the same time period.
This become even more problematic when you consider almost all endgame content are 8 man raids and if a WHM does a full regen rotation they can feasibly generate in excess of 1400 enmity in a single tic before a cure is even cast.
This causes even more issues if a party member is given regen before engaging a group of enemies. In the majority of instances while the tank is pulling the enemy or group of enemies, they gain a tic of regen and the monsters immediately run onto the mage, even if the tank is on full HP.
Even more confusing I was running an instance of Amdapor Keep today and died while regen remained on my tank. I returned to the beginning of the instance and a enemy monster I had slept awoke and immediately started trying to run to the beginning of the instance to attack me due to enmity generation from the ongoing regen.
I am unsure having not had the time to level SCH if the same issue exists with Whispering Dawn, but considering the amount of HoT abilities used by the healing classes I feel this is something that really needs to be addressed.
I have also submitted a bug report but I also wanted to post here as I have a feeling this is currently 'working as intended' and the entire ramifications of how much enmity this would generate has slipped past the development team.