Multipliers
Difficulty | Dmg Done | Dmg Taken | Legendary spawn rate ------------------------------------------------------------- V.easy | x 2 | x 0.5 | x 0.25 Easy | x 1.5 | x 0.75 | x 0.5 Normal | x 1 | x 1 | x 1 Hard | x 0.75 | x 1.5 | x 1.5 V.Hard | x 0.5 | x 2 | x 2 Survival | x 0.575 | x 3 | x 23
Not sure about the # for NPC's HP increase.
It also affect some other things, like the Healing speed/rate, spawn rate of legendary enemies and other similar effects. I don't have hard numbers for those though. There are no new mechanics on higher difficulty.