Dpsing and healing in Vanilla was completely different to how it is in retail.
Dpsers had to spam one or two abilities but also use auto attack timing or the recasting of a spell with pinpoint timing so you could make up for the visual lag in cast bars.
In retail dps is all just following a premade setup of cooldowns and watching your addons for the big red warnings to use those cooldowns.
Healers are similar, in Vanilla if alot of healing was needed on the group depending on your class you threw out a few hots or some low efficiency quick cast heals on appropriate targets and then re targeted the tank again and spammed high efficiency long cast heals but cancelled them at 80% cast if the tank was full hp while watching the raid for debuffs.
Just like dps in retail healing is all about following an addons orders that informs you when strong heals with cd’s are up and when to cast cooldowns to renew buffs on yourself to allow you to just keep spamming heals and hots.
Retail raiding took timing and knowledge of your classes basic functions and when to not just spam basic abilities or your mana pool will be dead in 20 seconds.
I’m a fan of addons but they changed the way the game was played and made top tier raiding entirely about who had the best gear setups and could follow orders on a screen to keep their buffs and debuffs up for longer.