well, going to put it here separated:
Zone Scaling: Best thing blizzard did. it was a damn time to implement it, since we outleveled a zone before reaching half of then.
mob HP increased: Disliked it, since we lost damage because of the nerf of heirlooms, it just made it longer to kill things, and for who dont have heirlooms, well.. taking forever.
Increased XP needed to level: Well, i am fine this, as long as there no mob HP increase.
And, of course, with the hp increase, we saw the unbalancing of class on leveling. While Warriors are rockin in killing mobs, monks are a pain to kill things. Shadow Priest too, have one at 60 and it take forever to kill things, even with full hierlooms. Class are so unbalanced at low levels that it makes worse to level up some classes really. and yes, we know, blizzard wont be fixing class low level issues. At legion, all class is fine to level tbh, since they are planned to work properly at lvl 100+.
and for thoses who thing making mob longer to kill will teach people how to play the class, i have a notice for you: you are wrong! the rotation you do from 10-100 is just basic rotation, and that even a dumb can do that. thing changes at lvl 110 because of artifact traits, artifact skill, talents, legendarys and trinkets. So this is a no excuse, you will still see people sucking at 110 on doing healing, dpsing at 110...