im leveling a shaman with swarm and vines and usually early on its a cakewalk but the fire/ice twin mutants you meet around lv 13 actually tanked a ton of damage and got me close to death. was specific abilities nerfed or was the game made harder? specifically the random bosses you face.
Yes.
"[Game]
Difficulties have been rebalanced to account for the ability to play up to level 100 on all difficulties. Normal difficulty is a little harder as you approach level 100 in all game modes to smooth out the transition to higher difficulties, while Elite and Ultimate difficulties are now easier at level 1-50 for new characters using Merits."
Depending on when you last played, the vanilla game is quite a bit harder than it used to be. Most enemies had their stats and skills altered to deal more damage.
Also Bosses that have rare Monster Infrequent gear always wear them in 1.2. This makes bosses like Voldrak (Troll just before Deadman Gulch) consistently much harder than before.
enemies deal a bit more dmg than before, + new sunder mechanic ofc, and has more hp, early game that HP difference is on appearance quite large (90% in some cases at low level) and then tapers off towards the end/higher level.
^In effect the hp increase doesn’t really have that huge an “effective” impact because it’s sorta just smoothing out a bit of powercreep,
the dmg difference however can be noticeable, or more noticeable if Sundered, specially if you weren’t used to covering your base defences before (resist/armour rating)
I play primarily Veteran so can’t really speak to the direct impact for Normal, but i was delighted when in act 1 Gutworm suddenly started to 1shot by chars for 1800-2500 dmg as i was still treating him pretty callously 
Certain other areas i also noticed i had to start treat the bosses with newfound respect, or char/build considerations.