First and foremost, Tomb Viper poison bug, that adds physical attack damage to every frame of the Tomb Vipers in Nihlathak’s Temple.
Gloam / Will O’Wisp / Burning Soul bug deals their supposed mana drain damage as lightning damage for their elemental attack, instead of their elemental attack damage.
Mana Burn bug, where monsters with mana burn drain 64 times more mana than the affix was supposed to deal; 4 times melee damage, but instead deals 256 times melee damage.
Monster elemental damage being able of critical hits against players.
Smite ignoring the attack routine is probably the biggest skill bug;
It was supposed to use the attacker’s attack rating and the defenders defense rating, but skips that part because of a bug.
Inferno and Arctic Blast dealing half damage – because the skill is separated into two rays, and monsters can only be damaged by one at a time – should obviously be fixed, but there is no guarantee, and it is fundamental.
…then, if they copy-paste mechanics without reworking them, there’s the case of Fend / Zeal bugs and Dodge / Evade / Avoid bugs – plus desynch issues relating to Dodge / Evade / Avoid.
The game will be notably different if these bugs are fixed.