Once Blizzard released the Burning Crusade characters wearing gear for those levels and at level 70 could 5 or 10 man what had been raids in WoW vanilla. And then by the end of Cataclysm you could solo Molten Core with most character types.

As a result you ended up in a world where 90% of the content was laughably easy, and rather discontinuous with respect to the original story lines. It became this weird easy sauce thing where you raced from level 1 to whatever the bottom level was of the current expansion, and then you worked your way up from there. But things you could get later could often be applied earlier (runes, enchantments, etc) making your gear massively overpowered which was "great" for leveling if you just wanted to get from 1 to whatever.

At one of the Blizcons the devs even said it was easier to do that then to try to create content that was engaging and that everyone could enjoy.

I'll be the first to admit that this was felt more by people who had been through the whole game than those who joined later, which is why I think folks playing through vanilla for the first time are so amazed by it.