Monsters are no longer vulnerable to elements.
They will not proc Explosion, Disintegration, Deep Freeze, Pestilence, or Venom due to Fire, Shock, Frost, Disease, or Poison attacks used against them.
Likewise, monsters no longer have resistance values reduced against damage dealt by these elements.
Monsters are no longer resistant to elements.
They can now proc Burning, Concussion, Chill, Diseased, or Poison due to Fire, Shock, Frost, Disease, or Poison attacks used against them.
Likewise, monsters no longer have resistance values raised against damage dealt by these elements.
There is no weakness to certain elements anymore. Also no immunity as well. This was changed in Summerset patch. This is taken from Summerset Patch Notes: