That insta-boost to level 90 is popamole as fuck. After the expansion goes live, you won't even be able to distinguish whether that lvl 90 running around just installed the game 5 minutes ago for the first time or someone who at least bothered to go through the leveling process. Until WoD when you saw a lvl 90 character you could figure it's not the most casual of players, now everyone will be lvl 90.
Because leveling in WoW was hard at any point? Most people were clueless about their class at every level cap - 60, 70, 80... how will that any different?
While leveling was never hard "hard", there certainly was more substance to it especially during vanilla. Respawn timers were faster, especially in mines/caves (Fucking kobolds), mob hp - your damage ratio was far more conservative, you didn't have access to AoE as non-mage (or if you did it was ineffective as fuck against mobs not intended to be AoEd), mobs were grouped up and if you wanted to be effective, you needed to use actual CC in outdoors, alone. Also they had spells that mobs typically don't have these days because they are "inconvenient" to the players, like heals and fears. Many times mistakes happened due to missed patrol, mobs running away and you had to get creative with the often limited toolkit you had.
Sure, none of that required any particular skill per se, just a bit of it spiced up with situational awarness. Regardless, it had far more going on for it than what there is currently, which is eyes off and spam the biggest number, typically that does damage 360 degree around you. Sometimes you might need to dodge giant DODGE THIS warning on ground.