Judgement of Salvation, and fuck that because it took up a debuff slot and only benefitted mages, warlocks and hunters, and also if you were a paladin, ret and meleeing the boss gtfo. It was healers that primarily had to manage mana. The maintenance of throughput, threat, and mana management was the whole thing. A strong tank meant you could just spam Healing Touch rank 4 as a druid with zero mana/threat issues. Mage would also OOM or pull threat regardless of mana regen if they were dumping spells on a boss.
At peak gear the buffs of shaman vs paladin came down to kings vs windfury/air (for hunters). Shaman also had the conflict of element between totem use - water was mana regen, healing, and fire resistance, air was windfury, air, tranquil...
It was a lot more complicated than a lot of flat statement folks ("one is clearly better") remember. Kings certainly helped early on by giving a lot more Hp and reduced threat risk of dps for nongeared tanks, but long term shaman was a lot more versatile in raids. I played both sides during vanilla, and wouldn't go back to alliance after playing horde.