I just finished the game. Why did people start referring to my monk as a "nephalem"? What the fuck is a nephalem? I thought I was paying attention...
Nephalem were the original offspring of angels and demons. The worldstone limited their powers till they became regular humans, however, with the worldstone gone their power have started to return, which is why the heroes from this game can tap into their powers.
The main reason everyone started calling your character a nephalem in the 4th act was because you were talking to angels and demons who don't see you as human, but instead see you as the nephalem (since that's what they refer to your race as).
@Dhutch: I thought that the dark wanderer was always Leorics son. They did retconn the character from the first game to be a named dude (even though you could have been a lady.)
Yeah. I'd beaten Diablo 1 as a Rogue, so I was bummed when the "Hero of Tristam" was unequivocally a dude. But it seems like Blizzard wanted from the outset for Diablo to fall to the "warrior" from that game.
They have explained what happened to the other characters from the first diablo though. As stated the Warrior became the Dark Wanderer. The Sorcerer became The Summoner and the Rogue became Blood Raven.