Wat? WotLK was the first expansion that had mass-scale catch up mechanics during its entire run. And they were introduced for the first time in late TBC.
The lessons of Vanilla and TBC were those of a grind... Grind bajillion consumables. Grind the same brainless bosses in MC for months to gear up your raid. Repeat that process in later tiers. Grind mats for resistance gear. Then there's the time consuming process of getting 40 players to raid in the first place. The difficulty of vanilla game from the constant gear checks that were overcome through grinding easier content. Today's 5 man dungeons have more varied and more challenging mechanics than the entirety of Naxx 40.