Self-sustain and healing abilities given to other roles
Other roles should feel like they contribute to helping the healer as some have more support than others and fills their job fantasy

Over simplified DPS rotation
Healers already struggle to do their 2 button rotation and in harder content where it is expected, having a more complicated rotation but having the same expectations would make the difference of a good healer and bad healer even greater. If you clear the tier on content or do ultimates it is most noticeable when you used to actually fail to meet damage checks because of the healers. Those healers would end up either not clearing until the party can carry them with gear or they end up becoming better.

There is optimization for healers at the very high end like astro giving 3 cards during div with lightspeed and 1 during odd minute but I tend to see astros just give cards on cd even in savage. You also have lily management so you can feed buffs as whm, double phlegma as sage, energy drain as sch during chain. Keep in mind they are revamping astro, giving whm more damage during burst and giving sage another damage button. There is complexity, it just matters how much you care about maximizing your toolkit. If you don't care about minmax/optimization then you really shouldn't care that much about your rotation being more complicated in the first place.

Homogenization of healer jobs
There's only so much you can do with healers since the game does not want you to be a heal bot and they also want you to be able to clear content with any 2 healers so how is whm/ast gonna mit deadly raidwides if they don't dip a little mitigation (wings/collective/exaltation/aquaveil/neutral/benison)

Excessive oGCD heals
It's not excessive if you do on early savage like 1-3 weeks, criterion, ultimates (the older ones u don't even have your full toolkit so it actually feels horrible because u need to mash those gcd heals). I do think there is room for SE to make more heal checks like spot healing and more raidwide damage but everyone hates healers more when they fail. It's the same feeling like a tank failing tank swap or dps actually being an actual floor dps where some or everyone will feel that mistake.

Lack of threat level in nearly all forms of content
There is threat levels in many dungeons, trials, savages, ultimates, criterions. I wouldn't expect every sprout to be pumping wall to wall in satasha but by the time they get out of arr they definitely get increasing difficulty in dungeons while not being so hard it stops them from doing the story. I've had wipes in dungeons because the healer died, no raisers and we would continue to wipe until the healer understood the mechanic or didn't get rnged with it.