Belial, The Lord of Lies, has two mouths and four eyes. The two mouths face in opposite directions (left and right of his "head"). Note where the extra eyes are.◊ Belial is "two-faced".
The Blizzcon 2010 demo of Diablo 3 showed that all the doors in Leoric's castle/torture chamber were about 15 feet high and 8 feet wide. Seems like it's just creative exaggeration, but Leoric's Ghost (and skeletal form from Blizzcon 2008) is way bigger than a human. Perhaps the doors were built with him in mind.
You may find it strange that the player character in Diablo 3 is able to identify items and do a Town Portal back to town without using any scrolls, any Mana, or have other characters identify items for you. But consider this...the player characters of the first game were human, so they needed all three methods to accomplish these feats. The players in the second game were either human or depowered Nephalem, so they needed these three methods like before. The player characters in the third game are unambiguously revealed to be fully-powered Nephalem, so they could identify items and use Town Portal as naturally as breathing, and render the three methods unnecessary!
Malthael in Reaper of Souls had done unwise things for the Archangel of Wisdom: namely absorbing the Black Soulstone into his body. It seems shocking yet players forget that in the Sin War Trilogy Malthael is called the Archangel of Death, not Wisdom. He may have claimed to be Wisdom to his followers, but he was always Death. It explains why he was called to by human souls. As Death the he's drawn to souls. This also explains why he couldn't find Wisdom in the Chalice of Wisdom after the Worldstone was destroyed: Tyreal was wiser than Malthael so it abandoned him just as the dead Angels in the Halls of Justice later lashed out at Tyreal in Storm of Light. As it saw Tyreal as more worthy of its Wisdom the Chalice of Wisdom gave him the answer he sought.
If Tyreal is the true Aspect of Wisdom than who's the true Aspect of Justice!?! Imperius of course! The End of Days prophecy says that Valor shall turn to Wrath and judging from Imperius's attempts to hide his true wrathful personality until provoked by Diablo which is a cowardly thing to do he was always Wrath instead of Valor(which is just another name for courage). What is Wrath(not to be mistaken for the Deadly Sin of Anger)? The divine hand of Justice!
It seems odd that Barbarians cannot carry two handed weapons with one hand like the Crusader can, but then you realize that the Barb-exclusive mighty weapons are simply exclusive to them because they are said to be far too heavy for anyone else to carry, crusaders included.
Greed, Baroness of the Treasure Realm isn't a Sin Lord according to Word of God so why is she an embodiment of Greed? Simple: It's because the Deadly Sin is called Avarice which is more specifically tied to money rather than treasure or other material possessions like Greed is and as such the Sin Lord would be the Sin Lord of Avarice not Greed. The Sin Lord of Avarice's realm is likely filled only with gold and no other treasures and thus would be less impressive than the Treasure Realm of Greed, Baroness of the Treasure Realm.
Captain Rumford suddenly vanishes when the Coven attack New Tristram and kill Cain. He doesn't reappear until the finale...as a ghost. Do the math.
In the larder of Bastion's Keep, you find Ghom's journal, which reveals that not only has he been eating people, but he's also been slaughtering people and feeding them to the unsuspecting denizens of the Keep. The hero and follower don't react to this, but unless they haven't been eating anything at all, they've been eating what everyone else has.
Sometimes you can find healing wells in the Halls of Agony, Leoric's personal torture chambers. Why are they there? Why are they there???!!!