(Unpopular opinion disclaimer: I guess I'm just a broken player who likes "broken" designs. It doesn't mean y'all are wrong though. I just want something different.)
I think I figured out why I like WHMs current kit (so I only want tiny buffs). It's the only healer actually playing a resource management game.
All the other healers just have so many free cooldowns, they barely even need to keep track of which ones are up. In Savage you plan what to do with your 2-/3-minute CDs and the rest is easily filled just by alternating Earthly Star and CO, or Physis and Kerchole. SCH at least loses damage if they're spamming Sacred Soil/Indom, and Whispering Dawn/Fey Blessing isn't quite enough in some situations.
WHM has slightly fewer cooldowns so they need to pay attention to them more. Burning a lily just for movement, or panic healing with lilies/medica 2 when Assize/Asylum could handle it if you planned ahead -- those lazy things will cost you in the long run. You pay in MP, and you pay in damage. WHM actually has to pay attention to MP efficiency (somewhat) and cooldown efficiency to manage the costs. And that optimization is fun.
The only semblance of difficulty on the other healers is keeping track of a larger number of abilities. But that's not really interesting if you aren't forced to use them all. For example, healing TEA on AST was fun cause my cooldowns were actually stretched thin. But that's the exception that proves the rule.
On the other hand, in P2S, I switched to SGE for one reclear and was able to heal it easily, first-time, forgetting to use Holos/Panhaima/Pnuema in several spots. Physis and Kerachole just cover way too much healing on their own. P3S and P4S would probably take a bit more experience/planning, but I know once I figure out where to use Panhaima/Pnuema the rest would fall easily into place. And I would pay nothing for it. There's no room for growth there.
On WHM, I'm trying to squeeze out every use of my cooldowns as possible, and taking inventory of every point of cure potency to avoid wasting anything, while still using Medica II on harder mechanics. Eventually my static (sorta casual) will all have BiS and we'll get more optimized, and I might be able to drop some more Medica II casts.
And to me, that's not a weakness or a punishment, it's a challenge and a reward. Every other healer getting everything for free just sounds cheap and boring.
Maybe Dragonsong Ultimate will shift the balance enough, so that WHM struggles too much, and the other healers struggle adequately.