Vanilla's content itself was a mess. You'd pick up a quest in Ashenvale at 20, walk to the clear other side of it, on foot, kill the mobs, walk walk walk all the way back and get the follow up. Which was a level 25 quest. Do that quest and the follow up is a level 30 quest. Unless you're going up 5 levels between quest turn ins you rapidly find yourself priced out of an area, forced to move onto another zone.
You'd do the same in the next zone then trek back to the first one to hit up the follow ups you left off at. It was convoluted and nuts and the only way to circumvent it was with Jame's leveling guide chartering the perfect route through manageable leveled content. You also had quests that required all sorts of obscure items that had you combing the world or the AH for just to get to the next follow up quest. Not talking legendary quest line here, just gold ol' XP. Cata basically fixed all of that, rectifying all the "logistical" woes of vanilla. But the mob stats were under tuned. A pity because compared to the logistical nightmare, retuning the numbers is an easier fix.