tl;dr - Empathy's abilities are met identically in most cases by the others, or outclassed by miles in others. Can Empathy get some minor tweaks/buffs to help it stand out as the "just heals" powerset?
After spending the last hour comparing the power sets and compiling a list of all the abilities (see the spoiler box in the first reply), it is kind of apparent that Empathy was really left behind in most cases. It was the "just heals" set for awhile, but a lot of the later (or newer) powersets left Empathy behind in the way DVDs surpassed VHS tapes.
To help minimize the post length, I am shortening the powerset names as shown here: Empathy -> E, Nature Affinity -> NA, Pain Domination -> PD, Thermal Radiation -> TR.
Here are some areas I noticed issues:
Single Target Healing - E's Absorb Pain heal value is only 25% better than PD's Share Pain, while dealing more self damage and granting no self buffs. E's Heal Other is identical in all respects (cost/cd/healing) to PD's Soothe and TR's Cauterize.
Group Healing - E's Healing Aura is identical to PD's Nullify Pain and TR's Warmth in every respect (cost/cd/healing). NA's Regrowth and Lifegiving Spores could easily outpace that with Bloom stacks over a 30 or 60 second window of combat.
Resurrection - E's Resurrect is a full heal with 90s of debt protection, but PD's Conduit of Pain is a full heal with massive self buffs for 1min and minor/negligible self debuffs (if playing support) for 1min. I assume debt protection isn't provided with Conduit of Pain as the tooltip doesn't mention it and the advanced tooltip shows nothing that indicates it.
Spammable/Toggle Buffs - E's Clear Mind is outclassed by PD's Enforced Morale (1) & TR's Thaw (2) which do the exact same thing with the same cost/cd, in addition to (1) 5% recharge reduction or (2) 10% cold resist and 80% movement slow resistance. TR's Thermal Shield and Plasma Shield provide effectively permanent resistance to smashing, lethal, fire, energy, negative energy, and cold (while also nearly maxing out fire resist without other buffs).
Cooldown Buffs - E's Regeneration Aura is much stronger in terms of regeneration than all others and E's Recovery Aura is helpful, NA's Wild Growth & Wild Bastion & Overgrowth can turn everyone in range into absolute monsters in nearly every respect with half the cooldown (8m20s on either of E's auras vs Wild Growth 3m45s, Wild Bastion 4m, Overgrowth 4m15s). E's Adrenaline Boost is the only thing that provides significant regeneration & recovery to a single target, but it's not spammable and not going to prevent a truckload of incoming (burst) damage.
Debuffs - Empathy does not have, nor does it need, debuffs. However, it's hard to overlook the significant advantage things like NA's Corrosive Enzymes, NA's Spore Cloud, TR's Heat Exhaustion, or TR's Melt Armor provide in terms of reducing incoming damage.
Additionally, in the later game, recovery is (mostly) moot as IOs/sets/etc make most characters fully self sustaining. With ambient healing, incarnates, and self abilities: the direct healing isn't even that useful anymore. What does empathy even bring to the table anymore?
I feel like Empathy could be made useful with relatively minor tweaks, not full redesigns or ability replacement(s). I feel like converting overhealing into minor absorption is the main path to success. Below are some minor suggestions that I believe could give Empathy a seat at the raid table.
Passive - Add a passive to Empathy that converts overhealing into a shield at some percentage (50%?) where only the most powerful applies, not cumulatively stacking.
In example: The next heal overheals 150 which adds 75 hp absorption, but the next heal overheals 80 which doesn't increase/reset the absorption value.
Direct Healing - Bump the direct healing amounts (Heal Other, Healing Aura) by 30% or more to stand above others in direct healing.
Absorb Pain - Remove self damage and just add absorption.
Recovery Aura - Half the cooldown and add a recharge reduction of maybe 25-30%.
Regeneration Aura - Half the cooldown and add healing received increase of 25-40% (reminder that the duration is only 40s, unlike NA Bloom's potentially unlimited duration). This also could interact with creation of absorption buffers.
Adrenalin Boost - Add a small resistance to all damage (10-20%?) as it thematically fits with an adrenaline surge in real life.
If you made it this far, thanks for reading and take care!