In retail, your character is a killing machine and the degree with which you pilot him determines his effectiveness. In Vanilla, your killing machine has auto-pilot, but you have to build the machine.
Vanilla was an entirely different genre of game. Similar to MTG, or Hearthstone, or DnD, a lot of the fun of Vanilla raiding came from planning and preparing. Vanilla was not an action game like modern WoW, nor was it trying to be. It was trying to bring the classic DnD/tabeltop style RPG to a real-time online environment.
There was a lot of depth to speccing and gearing your character. While there were plenty of cookie-cutter specs and stat guidelines, if you wanted to be exceptional - you had to be switching out gear for each boss trying to balance dps/healing with mana regeneration.
Contrary to popular belief, not every player was running the "1337 best PvE min/max talent spec". For example, if a druid in your guild already has improved MotW, there's little reason for you to also spend 5 talent points there. There was a lot of room for creativity, and building your character to have the desired amount of effectiveness in raids, farming, and PvP. Your character was very much your personal creation and a reflection of your playstyle.
Finally, a thing of note: Vanilla was much more than raiding, and it was ultimately a failure of the way 40-mans were designed that led to the "boring rotations". In 5-mans players were constantly using most of their spellbook, and situational awareness, reaction time, and general "skill" and knowledge of the game and your class would prevent wipes consistently (which were punishing and could easily break up a PUG).