I agree that WoW was far more charming back when there was more variation. Though it was more "casual" than Everquest (remembering that EQ had world boss fights that had to be fought in shifts because of how many hours it would take to beat them - like potentially multiple DAYS worth of HP bar...and I mean irl days with 70 people beating on them or whatever it was), it was still unpolished and unrefined. It eventually became a "themepark" probably late Wrath and into Cata, where the Devs wanted a far more curated experience, and that's when it started to lose its charm and eventually lost me. Like pre-Cata, Holy Priests would have many versions of the Heal Spell on their bar. This is because Heal Rank 3 would do less healing, but have a lower mana cost and maybe lower cast time. In fact, Lesser Heal, Heal, and Greater Heal were three names for the same thing. Lesser Heal had 3 ranks, I believe, with the last one at level 15 or so, and Heal rank 1 was basically a slight improvement on the same spell and picked up the track for 7 or 8 ranks before Greater Heal took over. But HPriests basically used the quirk of this system to make their own heals that they could tailor to the situation, using the right heal for the situation at hand.
...then Cata came and the Devs decided they new better, so there were no ranks, and there were just three kinds of basic heal every healer had - a medium cost, slow as Christmas cast big heal; a high cost, really fast, little heal; and a low cost, nearly as slow as Christmas, slightly larger heal. In addition to making healers heal like crap since the encounter designers forgot they were supposed to be going for a triage model, it also made healers samey, but perhaps worse, it took away player agency and all those little quirks. They eventually just did away with talent trees with that not really 100% true argument everyone used the cookie-cutter build, and then you were just hard-locked into a spec to do anything, which removed almost all of the quirks. The last ones were removed later, like when they split Feral and the new Guardian Druids.
WoW still had healer subroles - mainly tank healer vs party healer - but in trying to break that, they really just broke healers and never quite got them fixed to a way I liked before I quit playing.
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Shurrikhan - Honestly, I don't DISagree with you on all that. We've kinda got designed into a corner at this point.
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ForsakenRoe - That said, in WoW, this was once the case as well. Every 10 man in Wrath and Cata had either a Paladin or Disc Priest in a guaranteed slot as the tank healer. The other slot was taken by any one of the others as the raid/party healer. In 25 mans, it was GENERALLY expected that a raid team would have one of each healer. There was a little wiggle room on this, but having 5 healer slots tended to mean you did - Holy Paladin for tank healing, Disc Priest for kind of support healing, Resto Shaman for raid sustain with HoT rolling, Resto Shaman for raid burst healing, and Holy Priest for kind of "fill in" healing as it could do a little of all types. A lot of raid groups would carry 6 healers, and so when Fistweaving was added, they just had that as their 6th. You HAD to have them for the longest time because of their individual dispells, then they made it where 5 and 10 man content debuffs were all just the "Magic" type and they gave all the dispells the trait of dispelling their type (Paladin was Disease, Druid was Poison or Curse, I can't remember, Shaman the other, and Priest was...maybe Disease too, I don't remember, and there were some that had two types), and made them where they were for their one type + for magic, so they could all do the job in smaller group content (Priest had a slight advantage from AOE Dispell Magic), but in big raids, you were expected to have one of each and the Devs made debuffs of each type in encounters so if you didn't have one of each, you were kind of screwed.
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There were a lot of things they did differently back then.
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ASkellington - I've heard it said that the Pure/Barrier dichotomy doesn't work because Barrier Healers have to be able to heal (non-coordinated group content) even if there are 2x Barrier Healers. This ultimately makes Pure Healers redundant. Add to that the community's hyperfocus on damage > all and it means the community will largely pick Healers that do "enough" healing (all of them) and then pick the ones that buff damage after that. Leading to the AST/SCH meta.
Oddly, SCH/SGE was quite popular (mostly because they both heal by oGCDs and together they have a massive mitigation suit that they can chain to provide consistent mitigation for the party, which in turn can lead to safer prog runs and more consistent and reliable damage for farm runs) as an alternative. So this gives every healer a place other than WHM, because it has neither the buffs of AST nor the mitigation suit of SGE that clicks together like perfectly interlocking puzzle pieces with SCH.
So even now we don't have a system that's working. I hope the Pure/Barrier thing goes the way of the Main Tank/Off Tank thing. Either lean fully into it to make it work or abandon it. Half-assing stuff like that results in the system just entirely breaking down. And that's even if we ignore AST solo healing Ultimates. XD