Leveling gets easier with expansions, hierlooms, and LFD bonus gear.
After TBC they shifted more and more zooming you to current expansion content. Especially when you started not to have many people to group with to do elite quest… which were the hard ones.
Ironically post level squish and timewalking addition leveling got harder… especially at current expac, but it will never really be as difficult as it was when we had elite quests.
The hardest you can really look at this is probably Wrath questing level.